Tuesday, 17 February 2015

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Body's Thermoregulation during sleep

The temperature of both the brain and the body fall during NREM sleep.  The longer the NREM-sleep episode, the more the temperature falls. By contrast, brain temperature increases during REM sleep. The control of body and brain temperature is closely tied to sleep regulation.
Human beings are endotherms - able to thermoregulate - , that is, maintain their body temperature. Body temperature is regulated through a balance of heat absorption, production and loss. Human temperature must be maintained within a fairly small range, up or down from the resting temperature of 98.6. Temperatures above 104.9 degrees Fahrenheit or below 92.3 degrees generally cause injury or death.
Humans have two zones to regulate, their core temperature and their shell temperature. The temperature of the abdominal, thoracic, and cranial cavities, which contain the vital organs, is called the core temperature. Core temperature is regulated by the brain. The shell temperature includes the temperature of the skin, subcutaneous tissues, and muscles, and it is more affected by external temperature. The core is able to conserve or release heat through the shell.
When the core temperature is too high, blood vessels in the skin dilate and heat is lost through their walls. (This is hardly news to observers; in Ancient Greece Hippocrates speculated that sleeping bodies feel cool to the touch because blood flows away from the skin,) Sweat is also produced, which evaporates and lowers temperature. If a human is too cold, the blood vessels constrict, conserving heat. Blood is preferentially shunted to the internal organs and away from the skin and peripheral structures like limbs.
The hypothalamus regulates body temperature between 96.8 and 100.4 degrees Fahrenheit over each 24 hour cycle. During the normal human circadian rhythm, sleep occurs when the core temperature is dropping. Sleep usually begins when the rate of temperature change and body heat loss is maximal. The average adult’s lowest temperature is at about 5 AM, or two hours before waking time.
body temperature over time
Graph courtesy of National Center on Sleep Disorders Research
Many mammals lose significant thermal regulatory capacity during sleep. Some animals like squirrels go into a torpor state during sleep, in which their body temperature dips well below the normal level for hours at a time. However, most research to date seems to indicate that humans do not have significant difficulty thermoregulating during sleep.
In one study, subjects were exposed to a range of temperatures during sleep. Based on animal models, the researchers expected REM sleep to cause difficulty in thermoregulation, but the results showed that there was very little disruption of thermoregulation during REM and other sleep stages. The subjects shivered slightly in cold temperatures during sleep stages 1 and 2. Although skin temperature increased as the subjects were exposed to higher temperatures, their core temperature readings did not change.
A recent Dutch study shows just how important temperature is when it comes to sleep quality and fragmentation. Fitting human subjects with thermosuits, the scientists were able to lower skin temperature less than a degree Centrigrade without affecting core body temperature. The changes were dramatic. People didn't wake up as much during the night and the percentage of the sleep spent in stages 3 and 4 (deep sleep) increased. The effects were most pronounced in the elderly and in people who suffered from insomnia. A 0.4 C decrease in skin temperature caused a decline in the probability of early morning waking from 0.58 to 0.04.
The same researchers found that people with narcolepsy tend to have higher skin temperature when asleep, and also when awake. They speculated that that hypocretin (orexin) deficiency in the brains of narcolepics affects skin-temperature regulation. It was also found that increasing the skin temperature can promote sleep.
Other studies have showed different thermoregulatory responses of human subjects, depending on the sleep stage and temperature of the environment. In a different study of adult humans, thermoregulatory efficiency during REM sleep was fairly well maintained. However, thermoregulation was less efficient during Slow Wave Sleep (SWS). When subjected to different environmental temperatures, regulatory processes were affected. An overly warm or cool temperature disturbed sleep. REM sleep decreased, as did SWS to a lesser extent.
However, warmth beforehand improved sleep, especially SWS. In depressed patients, sleep is disturbed as well as body temperature rhythms. In these patients, a warm temperature before sleep might be helpful.
Here's an interesting fact: you don't sweat or shiver during REM sleep. Sleep researcher Jim Horne compares the REM non-thermal regulation period to that of normal functioning of babies, who neither sweat nor shiver even when awake. Babies control their body temperature, when it gets too cold, not by shivering but by use of so-called "brown fat" which is a type of adipose tissue well suited to generating heat. Adults have substantially less brown fat, adjusting for body weight, than babies do, but Horne thinks it is possible that adults use brown fat to keep from cooling too much during REM.
Some scientific papers characterize REM as "poikilothermic sleep". Poikilotherms are organisms like frogs in which the body temperature varies. You don't see this term used often with regard to human sleep, but it is technically accurate. The shortness of the

MANIPULATING YOUR BODY TEMPERATURE TO GET TO SLEEP

You really can't change your body temperature much without getting severly ill. It is very dangerous if you temperature goes more than a few degrees above or below normal. However, many find that cooling down helps them get to sleep. Why does a warm (but not hot) bath help so many get to sleep? Because it ends up cooling you down, especially as you dry off and the residual water on your skin evaporates. Recent research by Dutch scientists found that by increasing skin temperature the sleep quality in elderly people could be enhanced. People in the test wore heated thermosuits and with a slight (half a degree) increase in skin temperature were ale to increase the length of time spent in slow wave sleep and decrease incidences of waking.

Circadian Clock and Circadian Rhythms

Circadian Clock
This circadian rhythm controls alertness, sleep, hormone production,body temperature and organ function.  The clock is not an organ; it could be called an organ system or a body-wide synchronization of oscillators that exist at a cellular level.
The relationship between core body temperature and waking/sleeping times plays out this circadian rhythm. When the body temperature is dropping, it is easier to get to sleep. When it rises, we tend to wake up. That is why it is easier to sleep in cool rooms.

Circadian Rhythms
We live on a clock, whether we want to or not.  Not a man-made clock necessarily, but a natural clock that even our Stone Age ancestors followed.  The rhythms of our days are at least partly biological.  Physiological functions as well as social and cultural events occur in cycles.  Even in our modern technological world these cycles are important and measurable differences in abilities are everyday tasks (cognitive and physical) depend on the time of day and where the body is within its cycles.
It is important that we be aware of our rhythms and the rhythms of others.  How cranky or amiable people are can depend on where they are in the cycle.  Job performance varies depending on where people are on the cycle, and such dangerous matters as drowsy driving are of informed by circadian rhythms.
The body’s physiological processes differ considerably in how sensitive they are to circadian rhythms.  Some respond more to circadian clock changes and others more to the sleep-wake process.  A variety of mechanisms in the body keep it all together, and external cues from the environment entrain the body to the larger world.  The most important external cue is daylight, and temperature, smells (moreso in some animals than humans), and food intake tell the body where it is on the timeline.  Man-made, cultural cues are important, too.  These include work and school times, television and radio programs, and the activity of friends and family.  Sometimes man-made clocks clash with the body’s natural clock and this can result in circadian rhythm sleep disorders.
The processes for keeping it altogether are manifold and a triumph of evolution.  It is a combination of a top-down control with feedback and checks and balance from various organ systems.  The brain, as might be expected, is in more-or-less control.  In particular, an area of the brain called the suprachiasmatic nucleus functions as a master clock, although the control is not as tight as it is in say, the master clock of a computer system.
The SCN is a system of smaller oscillators.  The individual SCN neurons can move in different periods (time cycles) in the laboratory – outside the body.  But when the neurons are bundled together in the brain they oscillate together.  The communication and syncing between the neurons is not due to neurotransmitters but to electrical potential.  The overall cycle of the SCN is therefore an emergent property – an agreement among the various neurons that work together.
Daylight and darkness provide external cues to the body and sync the circadian cycle to Nature.  The mechanism for this synchronization involves light hitting the eye and sending a signal to the SCN.  Detailed study has found the rods and cones in the retina are only tangentially involved in this process.  The most important anatomical features are neurons in the retina called ganglion cells that directly communicate to the SCN.  The pigments melanopsin and cryptochrome appear to be involved in the ganglion cells and their response to light, although the process is not fully understood.
Now, what happens in the rest of the body?  Every human cell appears to have some oscillation, some circadian activity.  But left alone, they drift.  When studies in laboratory glassware, cells lose their rhythm.  One thing the body does on a system-wide basis is coordinate cycles.  The SCN functions as the master clock and physiological systems tie things together.  These systems include nerves as well as chemicals like cortisol and melatonin.  Different parts of the body respond and react in their own ways and not all systems adjust or entrain as well as others.  For instance, the liver is slow to adjust. Scientists found kidney cells have a clock of 24.5 hours and cornea cells have one of 21.5 hours   This partially explains the vague feelings of disruption people feel when they change time zones – each organ is adjusting one its own terms.
There is feedback from the rest of the body to the brain pacemaker.  This can also be in the form of nervous signals or hormone release.  Referred to as "peripheral circadian oscillators", these subsystems in the body outside the brain run on their own but are influenced by and influence the master clock.  Circadian cycles are quite complex.

Circadian Cycles and Sleep

Here’s an overly simplistic explanation of sleep: it’s part of the circadian rhythm of life and is hardwired into the biology of persons and animals. Your brain wants to sleep when it gets dark and wake when it is light.

That’s partly true, but it’s not the whole story. A more precise explanation is that the sleep cycle stems from an interaction between the circadian clock and a separate sleep-wake homeostatic process. The "sleep homeostat" is, roughly, an accounting of the amount of sleep you’ve experienced recently and a drive to return to balance. It causes the sleep drive to be based on how much sleep you’ve got in the past, and is directly related to the concept of sleep debt or sleep deficit. The sleep homeostat is similar to the hunger homoestat. If you haven’t eaten in a while, you’re likely to be hungry regardless of the time of day. If you had a feast at lunch, you may not be hungry come dinnertime. Likewise, if you’ve stayed awake all night, you’ll probably feel like sleeping in the morning, even if the Sun is up.

However, that’s not to say that the circadian cycle doesn’t matter. Cues such as daylight and regularly scheduled social and family activity have powerful influences on how sleepy or awake a person feels. These cues affect the internal clock. Disturbances of the normal circadian rhythmicity can result in serious health consequences, including psychiatric disorders, such as depression. When sleep patterns are pushed around these are called circadian rhythm sleep disorders. Nature magazine reports that some experts estimate half of adults have rhythms that are out of sync with their daily schedules.  This disconnect may not rise to the level of circadian rhythm disorder, but it can put sleep stress on people who have it.  A word for this phenomenon is "social jet lag".  

Blind people often experience sleeping problems because their retinas are unable to detect light and they don’t have the circadian cues of daylight and night. Shift workers try to run their lives out of sync with light and dark cycles and consequently have problems.

To reduce the effects of jet lag, therapists try to manipulate the biological clock with a technique called light therapy. They expose people to special lights, many times brighter than ordinary household light, for several hours near the time the subjects want to wake up. This helps them reset their biological clocks and adjust to a new time zone.

Disturbed circadian rhythms are correlated with many mental and physical disorders including sleep disorders. People with circadian rhythm disruptions are more apt to get metabolic syndrome and  gastrointestinal problems and other illnesses.

Circadian rhythms are ubiquitous in the animal kingdom, and are a cellular property. Neurons in a dish can act as clocks. The genes responsible for this cyclic behavior have begun to be identified. Clocks enable organisms to adapt to their surroundings. Although scientists currently believe that clocks arose through independent evolution and may use different clock proteins, they all share several regulatory characteristics. In particular, they are maintained by a biochemical process known as a negative feedback loop. Another hormonal cycle related to sleep is a reciprocal interaction of the neuropeptides growth hormone-releasing hormone and corticotropin-releasing hormone.

Stages Of Sleep

Usually sleepers pass through five stages: 1, 2, 3, 4 and REM (rapid eye movement) sleep. These stages progress cyclically from 1 through REM then begin again with stage 1. A complete sleep cycle takes an average of 90 to 110 minutes. The first sleep cycles each night have relatively short REM sleeps and long periods of deep sleep but later in the night, REM periods lengthen and deep sleep time decreases.
Stage 1
Stage 1 is light sleep where you drift in and out of sleep and can be awakened easily. In this stage, the eyes move slowly and muscle activity slows. During this stage, many people experience sudden muscle contractions preceded by a sensation of falling. Stage 1 is the beginning of the sleep cycle, and is a relatively light stage of sleep. Stage 1 can be considered a transition period between wakefulness and sleep. In Stage 1, the brain produces high amplitude theta waves, which are very slow brain waves. This period of sleep lasts only a brief time (around 5-10 minutes). If you awaken someone during this stage, they might report that they weren't really asleep.
Stage 2
In stage 2, eye movement stops and brain waves become slower with only an occasional burst of rapid brain waves. Stage 2 is the second stage of sleep and lasts for approximately 20 minutes. The brain begins to produce bursts of rapid, rhythmic brain wave activity known as sleep spindles. Body temperature starts to decrease and heart rate begins to slow.
Stage 3
When a person enters stage 3, extremely slow brain waves called delta waves are interspersed with smaller, faster waves. This stage was previously divided into stages three and four. Deep, slow brain waves known as delta waves begin to emerge during stage 3 sleep. This stage is sometimes referred to as delta sleep because of the slow brain waves known as delta waves that occur during this time. During this stage, people become less responsive and noises and activity in the environment may fail to generate a response. It also acts as a transitional period between light sleep and a very deep sleep. Bed-wetting and sleepwalking are most likely to occur at the end of this stage of sleep.
Stage 4
In stage 4, the brain produces delta waves almost exclusively. Stages 3 and 4 are referred to as deep sleep or delta sleep, and it is very difficult to wake someone from them. In deep sleep, there is no eye movement or muscle activity. This is when some children experience bedwetting, sleepwalking or night terrors. Most dreaming occurs during the fourth stage of sleep, known as rapid eye movement (REM) sleep. REM sleep is characterized by eye movement, increased respiration rate and increased brain activity. REM sleep is also referred to as paradoxical sleep because while the brain and other body systems become more active, muscles become more relaxed. Dreaming occurs due because of increased brain activity, but voluntary muscles become paralyzed. In 2008 the sleep profession in the US eliminated the use of stage 4. Stages 3 and 4 are now considered stage 3.
Slow wave sleep comes mostly in the first half of the night, REM in the second half.  Waking may occur after REM.  If the waking period is long enough, the person may remember it the next morning.  Short awakenings may disappear with amnesia.
In the REM period, breathing becomes more rapid, irregular and shallow, eyes jerk rapidly and limb muscles are temporarily paralyzed. Brain waves during this stage increase to levels experienced when a person is awake. Also, heart rate increases, blood pressure rises, males develop erections and the body loses some of the ability to regulate its temperature. This is the time when most dreams occur, and, if awoken during REM sleep, a person can remember the dreams. Most people experience three to five intervals of REM sleep each night.
Infants spend almost 50% of their time in REM sleep. Adults spend nearly half of sleep time in stage 2, about 20% in REM and the other 30% is divided between the other three stages. Older adults spend progressively less time in REM sleep.

As sleep research is still a relatively young field, scientists did not discover REM sleep until 1953 when new machines were developed to monitor brain activity. Before this discovery it was believed that most brain activity ceased during sleep. Since then, scientists have also disproved the idea that deprivation of REM sleep can lead to insanity and have found that lack of REM sleep can alleviate clinical depression although they do not know why. Recent theories link REM sleep to learning and memory.


Stage
Frequency
Amplitude (micro volts)
Waveform type
awake
15-50
<50
pre-sleep
8-12
50
alpha rhthym
1
4-8
50-100
theta
2
4-15
50-150
splindle waves
3
2-4
100-150
spindle waves and slow waves
4
0.5-2
100-200
slow waves and delta waves
REM
15-30
<50

The brain waveform during REM has low amplitudes and high frequencies, just like the waking state. Early researchers actually called it "paradoxial sleep".
The American Academy of Sleep Medicine has floated use of the term "Stage R" for REM sleep, but this new terminology has not caught on.


Stages
Waking
REM Sleep
NREM Sleep
Stage 0
Stage R
Light Sleep
Deep Sleep
Stage 1
Stage 2
Stage 3
Stage 4
Eyes open, responsive to external stimuli, can hold intelligible conversation
Brain waves similar to waking.  Most vivid dreams happen in this stage.  Body does not move.
Transition between waking and sleep.  If awakened, person will claim was never asleep.
Main body of light sleep.  Memory consolidation.  Synaptic pruning.
Slow waves on EEG readings.
Slow waves on EEG readings.
16 to 18 hours per day
90 to 120 min/night
4 to 7 hours per night



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