The Sleep Cycle

Sufficient and restful sleep is a human need as basic as food, vital to emotional and physical well being. In recent years, scientists have made great strides in identifying patterns and functions of brain activity in sleep.

The daily cycle of life, which includes sleeping and waking, is called a circadian (meaning "about a day") rhythm, commonly referred to as the biologic clock. Hundreds of bodily functions follow biologic clocks, but sleeping and waking are the most prominent circadian rhythms.

Light signals coming through the eyes reset the circadian cycles each day. The response to light signals in the brain is an important key factor in sleep and in maintaining a normal circadian rhythm.

Light signals travel to a tiny cluster of nerves in the hypothalamus in the center of the brain, the body's master clock, which is called the supra chiasmatic nucleus or SCN. This nerve cluster takes its name from its location, which is just above (supra) the optic chiasm. The optic chiasm is a major junction for nerves transmitting information about light from the eyes.

The approach of dusk each day prompts the SCN to signal the nearby pineal gland (named so because it resembles a pine-cone) to produce the hormone melatonin. Melatonin is an important hormone released in the brain that some experts believe is critical for the body's time-setting. The longer a person is in darkness the longer melatonin is secreted. Levels drop after staying in bright light. Research is ongoing to determine if high levels of melatonin cause sleep regardless of whether it is dark.

The sleep-wake cycles in humans are designed to produce activity during the day and sleep at night. There is also is a natural peak in sleepiness at mid-day, the traditional siesta time. The sleeping and waking cycle is approximately 24 hours. If confined to windowless apartments, with no clocks or other time cues, sleeping and waking as their bodies dictate, humans typically live on slightly longer than 24- hour cycles.

In sleep studies, subjects spend about one-third of their time asleep, suggesting that most people need about eight hours of sleep each day. Infants may sleep as many as 16 hours a day. Individual adults differ in the amount of sleep they need to feel well rested, however.

Daily rhythms intermesh with a number of biologic and physical factors that may interfere or change individual patterns. For example, the firing of nerve cells in the brain may be faster or slower in different individuals. Such differences are fractions of a second but they can cause variations in the type, timing, and duration of a person's sleep.

In women, their monthly menstrual cycle can shift the sleep/wake pattern. Changes in season or various exposures to light and dark often unsettle the sleeping pattern.

The importance of sunlight as a cue for circadian rhythms is dramatized by the problems experienced by people who are totally blind: they commonly suffer trouble sleeping and other rhythm disruptions.

Sleep consists of two distinct states that alternate in cycles and reflects differing levels of brain nerve cell activity. During a normal night's sleep, one progresses through these stages about five or six times:

Non-Rapid Eye Movement Sleep (Non-REM) sleep is also termed quiet sleep. Non-REM is further subdivided into three stages of progression:

  • Stage 1 (light sleep).
  • Stage 2 (so-called true sleep).
  • Stage 3 to 4 (deep "slow-wave" or delta sleep).

With each descending stage, awakening becomes more difficult. It is not known what governs Non-REM sleep in the brain. A balance between certain hormones, particularly growth and stress hormones may be important for deep sleep.

Rapid Eye-Movement Sleep (REM) sleep is termed active sleep and most vivid dreams occur during this stage. REM-sleep brain activity is comparable to that in waking, but the muscles are virtually paralyzed, possibly preventing people from acting out their dreams.

In fact, except for vital organs like lungs and heart, the only muscles not paralyzed during REM are the eye muscles. REM sleep may be critical for learning and for day-to-day mood regulation. When people are sleep-deprived, their brains must work harder than when they are well rested.

The cycle between quiet (NREM) and active (REM) sleep generally follows the same pattern. After about 90 minutes of Non-REM sleep, eyes move rapidly behind closed lids, giving rise to REM sleep. As sleep progresses the Non-REM/REM cycle repeats. With each cycle, Non-REM sleep becomes progressively lighter, and REM sleep becomes progressively longer, lasting from a few minutes early in sleep to perhaps an hour at the end of the sleep episode.

Seems pretty simple and basic, doesn’t it? After all, we don’t have to learn how to sleep – it’s something we automatically know how to do, but some people still have problems. Why?