First Glance

Organization of Human Sleep

Normal nocturnal sleep in adults displays a consistent organization from night to night. After sleep onset, sleep usually progresses through NREM stage 1 to 4 within 45 to 60 minutes. Slow wave sleep predominates in the first third of the night and comprises 15 to 25 percent of total nocturnal sleep time in young adults. The percentage of slow wave sleep is influenced by several factors, most notably age. Prior sleep deprivation increases both the rapidity of slow wave onset and its percentage of total sleep.

After the first slow wave sleep episode, the progression of NREM stages reverses. The first REM sleep occurs within 80 minutes after sleep begins, although REM latency shortens with advancing age. More rapid onset of REM sleep in a young adult may (particularly if less than 30 minutes.) suggests pathology such as endogenous depression, narcolepsy, circadian rhythm disorders, or drug withdrawal.

NREM and REM alternate through the night with an average cycle of 90 to 110 minutes. (‘the ultradian’ cycle). As the sleep period lengthens, the portion of each cycle composed of slow wave sleep decreases and that of REM sleep increases. Overall REM sleep is 20 to 25 percent of total sleep, while NREM stages (1and 2) constitute 50 to 60 percent (increasing in elderly subjects). (Table 1)

Table 1:

Behavioral Correlates of Sleep States and Stages

Age – Slow wave sleep is most prominent during childhood, decreasing sharply at puberty and across the second and third decades of life. After the age of 30 years, there is a progressive, almost linear decline in the amount of slow wave sleep. In the otherwise healthy elderly, particularly in males, slow wave sleep may be completely absent.

• In infancy, REM sleep may comprise 50 percent of total sleep time. REM occupies a more constant percentage of total sleep time than does slow wave sleep.
• Sleep onset is associated with marked decrease in perception of both auditory and visual stimuli and loss of consciousness.
• At stage 1, subjects may respond to faint auditory or visual signals without ‘awakening’. Furthermore, although memory incorporation appears to be inhibited at the onset of NREM stage 1, subjects aroused from that stage frequently deny having been asleep. The progression of subsequent NREM stage corresponds with increasing depth of sleep, measured by increasing threshold for arousal with a variety of auditory stimuli.
• Awakening from REM sleep is associated with a recall of vivid dream more than 80% of the time.

Effect of Sleep on Physiological Functions

• Cardiovascular function – changes include a decrease in blood pressure and heart rate during NREM, particularly during slow wave sleep. Cardiac dysrhythmias may occur selectively during REM sleep.
• Respiratory function also changes. Respiratory rate and minute ventilation decrease during NREM sleep and become variable during REM sleep. In addition, the cough reflex is attenuated or absent during sleep.
• Endocrine function also varies with sleep. Slow wave sleep is associated with secretion of growth hormone in young men, while sleep in general is associated with augmented secretion of prolactin. Sleep onset is associated with inhibition of thyroid stimulating hormone (TSH) and of the adrenocorticotropic hormone (ACTH)-cortisol axis.
• Sleep is also associated with alterations of body temperature.

Functions of Sleep

• It is the time for mind and body to recover
• Necessary for mental, physical and emotional well being
• Secretion of growth hormone necessary for growth and healing of tissues
• Stimulation of immune system
• Improves memory and cognitive functions
• Secretion of melatonin which is necessary for sleep, inhibits tumor from growing, prevents viral infection and has antioxidant properties.

Sleep Deprivation

Sleep as we have seen is as essential to human body as air and water. It serves as an important rejuvenating process. Lack of sleep or repeated periods of sleep loss leads to:

• Loss of concentration at work and home
• Lack of performance leading to decreased productivity
• Mood changes including increased sleepiness, fatigue, irritability, disorientation etc.
• Neurological effects such as, nystagmus (rapid movement of eyeball), hand tremors, slurring of speech.
• Autonomic effects such as, change in blood pressure, heart rate, respiratory rate etc.
• Loss of periodic pattern of excretion of growth hormone, noradrenaline and prolactin. There may be decreased growth hormone secretion leading to decreased or delayed growth.

All these effects in turn will lead to increased morbidity and mortality.