Sleep... 

Sleep is defined as a state of reduced motor activity (movement), lower response to sensory stimulation, adoption of postures such as lying down with eyes closed and easy reversibility (when compared to coma or hibernation). Sleep is essential for the normal functioning of all systems in the body. Sleep has direct effect on the individual's mental and physical health. Lack or poor sleep can affect the individual's ability to fight and endure sickness, cause irritability, poor concentration, impaired memory and decreases hand-eye co-ordination.

In the past, sleep was considered a passive state (where nothing happened), however it is now known as a dynamic process (brain activity continues during sleep). In humans cells produce and release proteins essential for growth (especially in children) and tissue repair during sleep. Sleep helps humans maintain optimal emotional and social functioning while awake by giving a rest during sleep to the parts of the brain that control emotions and social interactions. Sleep is a complicated process that is not fully understood.

Brain activity plays an important part in sleep (See sleep cycle for more information). Neurotransmitters (chemicals involve in nerve signaling) control whether the individual is awake or sleep by acting on nerve cells in different parts of the brain. The neurons located in the brain stem cause sleep by inhibiting other parts of the brain that keep a person awake.

The table below illustrates some of the effects of sleep deprivation.

Nervous system	Metabolic activity of the brain decreases significantly after 24 hours of sustained wakefulness Drowsiness, reduced concentration and memory Hallucinations and mood swings
Immune system	Reduced immune system function, measured by white blood cell count.
Cardiovascular system	Increased heart rate variability
Other	Decreased in body temperature Decrease release of growth hormone. Depression

How much sleep is enough?

Infants: 16 hours

Teenagers: 9 hours

Adults: 7 to 8 hours per night. Some adults may need 5 hours or as many as 10 hours per night.

Pregnant women: particularly during the first 3 months may need more sleep.

*** If you feel drowsy during the day or during boring activities you haven't had enough sleep***

Interesting fact

Animal studies have shown that sleep-deprived rats have a severely shortened life span and a breakdown in the immune system. The normal life span of rats is 2-3 years, however sleep deprived rats live for only 3 weeks. These rats develop abnormal low body temperatures and sores on their tails and paws.

Circadian Rhythms

Biological variations that occur in the course of 24 hours are called circadian rhythms. Circadian (from latin meaning around the day) rhythms are controlled by the body's biological clock. Many bodily functions follow the biological clock, but sleep and wakefulness consist of the most important circadian rhythm. The hypothalamus controls the sleep/wakefulness rhythm.

Light directly affect the sleep/wakefulness rhythm. Light is called a zeitgeber , a German word meaning time-giver, as it sets the biological clock.

The hypothalamus also controls body temperature. Body temperature increases during the day and decrease at night. Prolactin, testosterone and growth hormone have a maximal secretion at night.

Circadian rhythms can be affected by external stimulus, beeping of the alarm, timing of meals, crossing of time zones (leading to jet lag), and shift work. It usually takes several days for the body to adjust to the new time.

People who work at night or shift work, experience symptoms similar to those seen in people with jet lag. The individual's wake up time conflicts with powerful sleep-regulating cues (ie. sunlight) they become drowsy at work and even have trouble falling sleep during their off time. In this case, the biological clock wants to do one thing while the individual is doing something else. People that do shift work may have an increased risk of heart, gastrointestinal, emotional and mental problems. All of these problems may be related to the disruption of the circadian rhythm.

World record for sleep deprivation

Tony Wright, a 42-year old man from Britain, broke the world record for sleep deprivation. He stayed wake for 11 days or 266 hours. The previous record was held by Randy Gardner (264 hours) and was set in 1964.