A team of researchers conducted a study in which they recorded the daily rhythms of hormones and body temperatures in 24 healthy young and old men and women over a one-month period. The researchers concluded that our internal clocks run on a daily cycle of 24 hours, 11 minutes.
"The variation between our subjects, with a 95 percent level of confidence, was no more than plus or minus 16 minutes," said Charles Czeisler, professor of medicine at the Harvard Medical School. "[That's] a remarkably small range."
"These data reveal that the human circadian pacemaker is as stable and precise in measuring time as that of other mammals," notes Richard Kronauer, Gordon McKay Professor of Mechanical Engineering.
The Harvard team had their subjects go to bed four hours later each day, effectively creating a 28-hour day. This strategy disconnected the biological pacemaker from clock time. "The 28-hour cycle distributed light exposure, sleep and wakefulness, work and play evenly around the biological clock," explains Czeisler. "The men and women did not get light exposure at the same time each clock day. Instead, they experienced a six-day week in which light and dark occurred at different times each day."
This altered schedule freed their internal clocks from the sleep-wake cycle and allowed them to tick at the natural period. Despite six-day weeks, their body temperatures and hormone secretions went through seven cycles every week. Sleepiness was tied to a drop in core body temperature and to an increase in melatonin, a hormone secreted by the pineal gland and sold over-the-counter as a sleeping pill.
Czeisler's crew also measured changes in cortisol, a hormone involved in metabolism and other basic body functions. Cortisol naturally drops to its lowest levels at bedtime and reaches its highest point during early waking hours.
The result was clear. No matter when the subjects went to bed or got up, and whatever they did while awake, body temperature and hormones rose and fell on an average cycle of 24 hours and 11 minutes.
Stages of Sleep
Human sleep has been described as a succession of five recurring stages: four nonrapid eye movement (REM) stages and one REM stage. A sixth stage, waking, is often included. Waking, in this context, is actually the phase during which a person falls asleep. REM sleep is marked by extensive physiological changes, such as accelerated respiration, increased brain activity, eye movement, and muscle relaxation. People dream during REM sleep, perhaps as a result of excited brain activity and the paralysis of major voluntary muscles.
Sleep quality changes with transition from one sleep stage into another. Although the signals for transition between the five (or six) stages of sleep are mysterious, it is important to remember that these stages are, in fact, discretely independent of one another, each marked by subtle changes in bodily function and each part of a predictable cycle whose intervals are observable. Sleep stages are monitored and examined clinically with polysomnography, which provides data regarding electrical and muscular states during sleep.
The waking stage is referred to as relaxed wakefulness, because this is the stage in which the body prepares for sleep. All people fall asleep with tense muscles, their eyes moving erratically. Then, normally, as a person becomes sleepier, the body begins to slow down. Muscles begin to relax, and eye movement slows to a roll.
Stage 1 sleep, or drowsiness, is often described as first in the sequence, especially in models where waking is not included. Polysomnography shows a 50 percent reduction in activity between wakefulness and stage 1 sleep. The eyes are closed during stage 1 sleep, but if aroused from it, a person may feel as if he or she has not slept. Stage 1 may last for five to 10 minutes.
Stage 2 is a period of light sleep during which polysomnographic readings show intermittent peaks and valleys, or positive and negative waves. These waves indicate spontaneous periods of muscle tone mixed with periods of muscle relaxation. Muscle tone of this kind can be seen in other stages of sleep as a reaction to auditory stimuli. The heart rate slows, and body temperature decreases. At this point, the body prepares to enter deep sleep.
Stages 3 and 4
These are deep sleep stages, with stage 4 being more intense than stage 3. These stages are known as slow-wave, or delta, sleep. During slow-wave sleep, especially during stage 4, the electromyogram records slow waves of high amplitude, indicating a pattern of deep sleep and rhythmic continuity.
The period of non-REM sleep (NREM) is comprised of stages 1-4 and lasts from 90 to 120 minutes, each stage lasting anywhere from 5 to 15 minutes. Surprisingly, however, stages 2 and 3 repeat backwards before REM sleep is attained. So a normal sleep cycle has this pattern: waking, stage 1, 2, 3, 4, 3, 2, REM. Usually, REM sleep occurs 90 minutes after sleep onset.
Stage 5, REM
REM sleep is distinguishable from NREM sleep by changes in physiological states, including its characteristic rapid eye movements. However, polysomnograms show wave patterns in REM to be similar to stage 1 sleep. In normal sleep (in people without disorders of sleep-wake patterns or REM behavior disorder), heart rate and respiration speed up and become erratic, while the face, fingers, and legs may twitch. Intense dreaming occurs during REM sleep as a result of heightened cerebral activity, but paralysis occurs simultaneously in the major voluntary muscle groups, including the submental muscles (muscles of the chin and neck).
Because REM is a mixture of encephalic (brain) states of excitement and muscular immobility, it is sometimes called paradoxical sleep. It is generally thought that REM-associated muscle paralysis is meant to keep the body from acting out the dreams that occur during this intensely cerebral stage. The first period of REM typically lasts 10 minutes, with each recurring REM stage lengthening, and the final one lasting an hour.
The five stages of sleep, including their repetition, occur cyclically. The first cycle, which ends after the completion of the first REM stage, usually lasts for 100 minutes. Each subsequent cycle lasts longer, as its respective REM stage extends. So a person may complete five cycles in a typical night's sleep. http://www.sleepdisorderchannel.com/stages/
Benefits of Rapid Eye Movement (REM) Sleep
REM sleep is needed because it triggers the ejection of growth hormones in youth. Deep sleep also allows protein, the building block required for cell growth, to break down and helps heal the destructive effects of ultraviolet rays and stress, which means that "deep sleep may truly be beauty sleep." Sleeping a restful seven to eight hours a night may prevent seizures and seizure spreading in epilepsy patients, conserve energy, increase the fight of bacterial infections, such as colds and the flu, increase overall health, and be a therapeutic agent for those undergoing depression. More important than receiving restful sleep is completing these hours strictly at night and in the early morning.
The benefits of sleep overall include: increased concentration, a positive attitude, healthier skin, more energy, relaxation and alertness, less stress, and increased brain activity. (National Institute of Neurological Disorders and Stroke, "Brain Basics: Understanding Sleep," http://www.ninds.nih.gov/disorders/brain_basics/understanding_sleep.htm#dreaming).