Researchers at the Max Planck Institute for Psychiatry in Munich, Germany (Jurgen Zulley and Scott Campbell), who isolated subjects in an underground bunker with no way to judge time outside of their environment (and with no music, writing, reading, alcohol, coffee or tea) found in 1987 that there were four-hour rhythms---periods of increased readiness to sleep---in addition to night sleep. Daytime sleeping occurred most often at 1 p.m., but shorter naps also took place at 9 a.m. and 5 p.m. Those in midday sleep for more than 90 minutes had deeper sleeps, followed by periods of dreaming.

      Richard E. Kronauer, professor of mechanical engineering at Harvard, found that people chose nap times at 5:30 p.m., slept for three hours, stayed up for eight hours and slept another four hours. In essence, their days were split in half.

      Thomas Wehr, chief of the clinical psychobiology branch of the National Institute of Mental Health, believes data indicates that the natural sleep pattern is a couple of hours in the daytime and five hours at night. Humans appear to be genetically programmed to nap.

      Psychologist Scott S. Campbell of the Institute for Circadian Physiology in Boston (formerly at the U. of California at San Diego), along with Irene Tobler, discovered napping behavior in 160 different species of animals, from insects to primates. All creatures except humans (due to cultural and industrialization factors) break up sleep into several discreet phases per day, or doze lightly while doing something else. Campbell believes that "we go against nature when we sleep just once in twenty-four hours". Enforced night sleep disrupts the natural tendency toward nodding off during the height of the day.

      Research indicates that reactions to allergens and many disease states are strongly linked to internal circadian rhythms.
      Asthma occurs more often at night than during the day, not only because there is more pollen or dust in the air in the evening, but because of fluctuations in immune system defenses.

      The threshold for pain is lower at night than it is in the afternoon. Migraine and muscle headaches usually begin in the early hours of the morning. the onset of fever from a viral infection occurs mainly during the late afternoon and evening, between 2 and 10 p.m. Onset of fever from bacterial infection occurs mainly during the morning, between 5 a.m. and noon. White blood cells, which play a large role in the immune response, are more active during the evening than at night, etc.

      Since the small proteins that enhance immune function are also sleep-inducers, is the built-in tendency to nap associated with general immune system enhancement; modification of hypertension? Are the four-hour rhythms/periods of increased tendency to sleep associated with pulses in increased immune system efficiency?
      It would appear so. The muramyl peptides are associated with deep, slow-wave sleep. David Dinges, a sleep researcher at the U. of Pennsylvania, has determined that during naps, MOST SLEEP IS IN THE DEEPEST STAGES, CHARACTERIZED BY SLOW, REGULAR BRAIN WAVES. Relatively little of a nap is spent in the phase of sleep during which most dreaming occurs.

      A study by the U. of Athens Medical School looked at Greeks, half of whom nap, half who don't. They compared men hospitalized for coronary heart disease with men hospitalized for other reasons and concluded that Greek men who nap half an hour or more every day are 30 percent less likely to develop a cardiac condition than those who don't nap.
[ It's a widespread custom in Japan to allow night-shift workers to have naps to break up their time on the job. ]

      Most afternoon naps are between 30 and 90 minutes. Naps under 15 minutes are uncommon. Naps of just a few minutes reach only the first stage of sleep. Slow-wave sleep doesn't occur until later in the cycle. Psychiatrist Martin Orne of the U. of Pennsylvania and Institute of penn. Hospital notes that napping should be practised "prophylactically" (!). "Nap *before* you're tired."

      On the average, naps begin about 12 hours after the middle of the main period of sleep. Someone who slept from midnight to 6 a.m. would be most highly primed for a nap around 3 p.m.