Features of NREM sleep:

Stage-1: It is considered to be a transition state between wakefulness and sleep. When a person closes his eyes, NREM sleep begins and is followed by several sudden sharp muscle contractions in leg, then relaxation .This period lasts for 3-12 minutes.

Stage-2: In this stage, frequency of the EEG decreases and amplitude increases. A series of high frequency waves known as sleep spindles interrupt the theta waves of this stage and they last for 1-2 seconds. Thalamic and cortical neurons interact and generate these waves. High amplitude forms called k-complex are shown during this stage in EEG traces. This stage constitutes about 50% of total time in sleep each night in adults.

Stage-3: This is the stage from moderate to true deep sleep (sws).A delta wave first appears, sleep spindles and k-complexes occur and this stage lasts for 10 minutes during the first sleep cycle and represents only 7% of the total night’s sleep.

Stage-4: This is the deepest NREM sleep. The EEG trace is dominated by delta waves and overall neutral activity is at its lowest. Breathing, heart rate, B.P and brain’s temperature are all reduced under the influence of para sympathetic nervous system. This is also the stage in which children may have episodes of somnambulism or sleep walking and night terrors.

REM sleep:

It is called dream sleep or paradoxical sleep. It constitutes about 20-25% of total sleeping time. During REMS memory is refreshed. A person will experience 4 to 5 periods of REM sleep during night. EEG recording of neural activity during REM shows almost the same tracing as to that during this period.

Effect of infection on sleep:

Sleep deprivation may produce varying effects ranging from lowered immune system, memory, gain of weight. During an infection, the brain activates the peripheral immune system. The neurons in the brain which regulate sleep are also related to the immune system. The brain functions by bringing a change in physiological processes to fight off the bacterial and viral infection. One of such approach is release of cytokines which also found to influence the increased sleep during infections. During an acute infection, the NREM increases. This may also be because of increase in concentration of IL-beta

Common cold⁽⁶⁾ is one of the most simple of infections of which everybody are aware of. During infection we feel uncomfortable may be of the breathing problem or fatigue or urge to sleep. But, when do we feel better (less discomfortable) that is during morning, after a good night sleep. This suggests sleep to be a better alternative than medicines during cold. The conditions degrade eventually and gets worse by evening. Short naps during the day may prove helpful which may be because during sleep the immune system gets activated or reenergized to fight against the infection and this is shown by few evidences.

Evidence to show an increase in sleep during infection:

Several experiments were conducted on both animals and humans but it is difficult to integrate the results as the immune parameters vary from species to species.

Animals:

A change in sleep patterns occurred when toth.et.al conducted a range of experiments by infecting animals with various micro organisms like viral infection of mice with influenza, fungal infection of rabbits with candida albicans and protozoan infection of rabbits(fig-3) with trypanasoma brucei⁽⁵⁾. Some animals specifically showed an increase in SWS duration and amplitude in a diseased condition called trypanosomiasi. The release of parasites in to blood stream is associated with increase in SWS although over all there is a decrease in SWS.

Fig-3: experimenting on rabbits by infecting them with micro organisms

Humans:

Viral infection with influenza virus causes sleep alterations although the quality of sleep is unaffected. A decreased sleep duration was observed on infection with rhinovirus. Administration of animal products like LPS causes a decrease in REM sleep but increases SWS duration, increases body temperature, heart rate, level of cytokines (TNF).

In a certain case called human African trypanosomniasis sleeping sickness both sleep disturbances and immune disorders occur. It was found out that the level of disturbance is directly proportional to the severity of illness.

In some cases of HIV, an increased duration of SWS during the latter half of the night occurs. But it is not clear whether all the infections cause an increase in sleep time. For example, in infants with mild respiratory tract infections, they did not show any increased sleep time or altered sleep proportions.

Role of cytokines in sleep:

Certain group of proteins called cytokines^{(9, 11,
12, 13)} present in our body act as connecting link between immune system and central nervous system. These cytokines concentration increases during night and evidences have shown that they are included in normal sleep wake regulation. To know their effects on sleep almost nearly 20 proteins have been studied among which interleukin-1 (IL-1) and tumor necrosis factor (TNF)⁽¹⁰⁾are found to influence the NREMS of mammals.

It has been observed through molecular genetics, electro physiological and biochemical studies that a reduction in NREM sleep occurs if the IL-1 and TNF systems are antagonized and also an increase in NREM sleep occurs when the body experiences antigenic challenge which may be due to the increased concentration of IL-1and TNF. These cytokines are identified as two types, proinflammatory (type-1) and anti-inflammatory (type-2). The receptors for these cytokines are located on the hypothalamic nuclei and brainstem. Older people generally suffer from less amount of sleep. One of the reasons is the increased levels of IL-6 and cortisol. They also influence ACTH, GH secretion.

Scientific studies have revealed that IL-2, IL-15, IL-18, IFN-alpha and IFN-gamma usually increase duration of sleep and IL-4, IL-10 and IL-13 inhibit spontaneous sleep. Mostly pro-inflammatory cytokines promote sleep and anti-inflammatory cytokines tend to inhibit.

Certain cytokines like IL-1 and TNF act on certain regions of brain which also regulate sleep. Sleep active neurons present in the preoptic area of the anterior hypothalamus are stimulated by both IL-1 and TNF.

IL-1 enhances adenosine levels in hippocampus and prostaglandin D2 (PGD2) synthesis both of which (ADENOSINE and PGD2) are known as sleep regulatory substances. The IL-1β (fig-4) and TNF-α are involved in regulation of sleep even without infection.

There exists an innate relation between sleep and other systems in our body like nervous system, immune system, endocrine system, circadian system etc.

SLEEP AND ENDOCRINE SYSTEM:

The endocrine system was found to play an important role as a connecting link between immune system and sleep through harmones. Hypothalamus produces corticotropin releasing harmone (CRH) a potent inducer of waking. Another important harmone is Growth harmone releasing harmone (GHRH). It promotes NREM sleep and its concentration was found to be highest in night. Growth harmone enhances tissue growth and protien anbolism and its synthesis is promoted by GHRH and Ghrelin. Cortisol inhibits the synthesis of IL-1 and few cytokines where as GHRH promotes it.

If GH secretion is supressed, it can lead to sleep supression. Eg:- Insulin like growth factor (IGF-1).An increase in cortisol might be the reason for disrupted sleep causes in adult. A decrease in SWS was observed when cortisol levels are increased. Prolactin is also found in its highest concentration during sleep. Somatostatin found in REMS, reduces NREMS and GH. Vasopressin was found to be useful in increasing REM, SWS and sleep time. In a study on rabbits, they are injected with 1nmol/kg centrally and it caused an increased time in NREMS i.e. a period of 53 minutes (during 1^st 6 hrs of injection).

CIRCADIAN SYSTEM:

There is an internal clock in our body which controls our sleep wake cycle. This circadian rhythm is modulated by hypothalamus. To be mentioned specifically it is called suprachiasmatic nucleus. These rhythms are light influenced. This can be one of the reasons for blind people to experience lifelong sleeping problems as their retinas are unable to detect light. One of the important aspects of sleep is circadian rhythmicity. If the immune system components are varied by light cycle it might support a link between immune system and sleep. During sleep, levels of lymphocytes and monocytes reach maximum and are lowest after waking. At night plasma TNF levels increase but it has been observed that during a sleep disorder like OBSTRUCTIVE SLEEP- APNOEA, the circadian rhythm of TNF was found to be disrupted. Also in rats it was shown that the magnitude of increase in SWS in response to IL-beta is dependent on circadian phase of administration.

EFFECT OF SLEEP DEPRIVATION ON IMMUNE SYSTEM⁽⁴⁾:

During sleep, every physiological variable changes relative to wakefulness, so it is not possible to isolate sleep as a separate variable and study its effects .So the animal is sleep deprived and its effect on the immune system are studied to understand the complex relation of sleep and immune system.

A protein known as Dimuramyl peptide accumulates in the body in the spinal fluid especially, when the animal is sleep deprived. The protein is not synthesised by the body and is secreted by bacteria. This indicates that sleep deprivation enhances bacterial growth. Not only that, they are also found to increase the NREM sleep. To get a better view of cytokines role in sleep, study of effects of sleep deprivation might be useful.

Experimental studies have shown that a change in the immune cell number occurred after sleep deprivation. They concluded that T-helper cells and NK cells decrease in number after 40 hrs of sleep deprivation and their number increases after 64 hrs deprivation which shows the importance of duration of sleep deprivation. Furthermore studies revealed controversial results. With shorter periods of sleep deprivation, the cell lytic activity of natural killer cells has decreased but the same activity increased during longer periods of sleep deprivation. Another study showed that in subjects who were sleep deprived for 72 hrs, a decrease in phagocytosis occurred.

A study in humans reported a decrease in the level of immunoglobulins on sleep deprivation, but this result has not been reproduced.

The cytokine IL-6 levels increase on onset of sleep. When sleep is delayed the concentration does not rise, it rises only when sleep begins. So if a person is sleep deprived, he must have normal levels of IL-6, on the contrary it was found that there is an increase in concentration of IL-6 than the normal individuals. This develops an urge to sleep indicating that cytokines play a part in promoting sleep.

Hormones particularly are found to be non-influential regarding the immune changes occurring with sleep deprivation. In an experiment conducted on mice, they are immunized against infection with influenza virus and then sleep deprived. When they are attacked by virus, they did not clear the virus efficiently. They acted similarly as that of unimmunised mice, but on further experiments these results are not reproduced. Another contrary effect observed is that in non-immune mice .sleep deprivation decreased the progress rate of influenza virus along respiratory tract.

When extreme sleep deprivation is considered in rats which were sleep deprived for 21 days, they developed Cachenia and Septicaemia where as in humans sleep deprivation did not show an increase in infections due to several reasons.

In another study, effects of sleep deprivation is studied by measuring the amount of stress produced .It was found that rats which are REMS deprived develop stress harmone response like increased corticosterone levels and decreased testosterone levels. These responses are similar to that of the rats which are given foot shock, a potent stressor. Stress ulcers are observed in rats with sub acute sleep deprivation.

In an experiment, certain subjects are subjected to sleep loss of 15, 39 and 63 hours and then made asleep. During deprivation an increase in the levels of IL-1β was observed and on recovery a decrease than the normal baseline levels was observed. The levels of IL-6 fluctuated during sleep deprivation and decrease on recovery.

In another study of human volunteers, they provided an extensive sleep loss in simulated spaceflight conditions, than increase in soluble TNF-α receptors and IL-6 was observed.

In order to find out the role of cytokines in sleep, certain cytokines or certain cytokine receptors were blocked and the effect it produces on sleep were observed. A decrease in spontaneous REMS was observed due to the absence of type-1 IFN receptor but no effect was produced on NREMS.

Both acute and chronic intermittent sleep deprivation exacerbates colonic inflammation. This is shown by experimenting on a mouse model of colitis. It worsens the inflammation and delays recovery.

SLEEP DISORDERS:

In few clinical conditions both disordered sleep and disordered immune functions occur which provide evidence of their relation.

When patients having allergy are observed they have a high concentration of serum levels of IL-1β, IL-4, and IL-10 when compared with patients who are not having allergy. It is also observed that in these patients, there is a faster sleep onset, shorter duration of REM sleep. In alcoholics an increase in REM sleep and loss of SWS is observed, along with this a decreased nocturnal IL-6 production and NK-cell activity is seen. In diseased state conditions like Fibromyalgia-disordered sleep occurs with sleep disturbances in NREMS .Along with it infectious Mononucleosis also occurs. Experimental studies have shown that in patients suffering from depression and patients suffering with sleep disorders-there is an increased serum levels of IL-6 and IL-8 and when faced by microbial challenge-lymphocytes response is greater than normal in both conditions .Scientific studies have shown that chronic insomnia was related with a reduction in the number of CD3†, CD4†CD8† T-cells and a decrease in the response of Natural killer cells.

CONCLUSION:

Sleep and Immune system are very much related to each other. Mainly many cytokines like interleukins and TNF influences REM and NREM sleep. Most of the proinflammatory and anti inflammatory cytokines regulates sleep by means of regulating adenosine and PGD2 levels. Even the endocrine system and circadian system also have relationship with sleep. The sleep deprivation causes a lot of irregularities in the levels of cytokines as well as hormones.

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Received on 03.12.2009

Accepted on 15.01.2010

Research J. Pharmacology and Pharmacodynamics 2(2): March –April 2010: 148-152