question: How does our body know the difference between a nap and a full nights sleep?
comment: Basically your brain has a 24 Hour cycle clock which relies on certain aspects to help tell what time it is using hormones and different signals. “A part of your hypothalamus (a portion of your brain) controls your circadian rhythm. That said, outside factors like lightness and darkness can also impact it. When it’s dark at night, your eyes send a signal to the hypothalamus that it’s time to feel tired. Your brain, in turn, sends a signal to your body to release melatonin, which makes your body tired. That’s why your circadian rhythm tends to coincide with the cycle of daytime and nighttime” [ URL_0 ]( URL_1 ) Edit: Summary added. Edit 2: I hope this helped, but I realise it does not fully answer the question from OP although a rhythm or cycle could help explain the difference between napping and night sleep.
passages: According to EEG measurements collected by Dr. Stampi during a 50-day trial of polyphasic ultrashort sleep with a test subject and published in his book "Why We Nap", the proportion of sleep stages remains roughly the same during both polyphasic and monophasic sleep schedules. The major differences are that the ratio of lighter sleep stages to deeper sleep stages is slightly reduced and that sleep stages are often taken out of order or not at all, that is, some naps may be composed primarily of slow wave sleep while rapid eye movement sleep dominates other naps.
   A Multiple Sleep Latency Test (MSLT) is often performed during the entire day after polysomnography while the electrodes and other equipment are still in place. The patient is given nap opportunities every second hour; the test measures the number of minutes it takes from the start of a daytime nap period to the first signs of sleep. It is a measure of daytime sleepiness; it also shows whether REM sleep is achieved in a short nap, a typical indication of narcolepsy.
   According to the book, in a sleep deprived condition, measurements of a polyphasic sleeper's memory retention and analytical ability show increases as compared with monophasic and biphasic sleep (but still a decrease of 12% as compared with free running sleep). According to Stampi, the improvement is due to an extraordinary evolutionary predisposition to adopt such a sleep schedule; he hypothesizes this is possibly because polyphasic sleep was the preferred schedule of ancestors of the human race for thousands of years prior to the adoption of the monophasic schedule.
   NASA, in cooperation with the National Space Biomedical Research Institute, has funded research on napping. Despite NASA recommendations that astronauts sleep eight hours a day when in space, they usually have trouble sleeping eight hours at a stretch, so the agency needs to know about the optimal length, timing and effect of naps. Professor David Dinges of the University of Pennsylvania School of Medicine led research in a laboratory setting on sleep schedules which combined various amounts of "anchor sleep", ranging from about four to eight hours in length, with no nap or daily naps of up to 2.5 hours. Longer naps were found to be better, with some cognitive functions benefiting more from napping than others. Vigilance and basic alertness benefited the least while working memory benefited greatly. Naps in the individual subjects' biological daytime worked well, but naps in their nighttime were followed by much greater sleep inertia lasting up to an hour.
   Why We Nap
  
  Why We Nap: Evolution, Chronobiology, and Functions of Polyphasic and Ultrashort Sleep (1992) is a book edited by Claudio Stampi, sole proprietor of the Chronobiology Research Institute. It is frequently mentioned by "polyphasic sleepers", as it is one of the few published books about the subject of systematic short napping in extreme situations where consolidated sleep is not possible.
   Section::::Correlates of sleep.
  
  One of the important questions in sleep research is clearly defining the sleep state. This problem arises because sleep was traditionally defined as a state of consciousness and not as a physiological state, thus there was no clear definition of what minimum set of events constitute sleep and distinguish it from other states of partial or no consciousness. The problem of making such a definition is complicated because it needs to include a variety of modes of sleep found across different species.
   For years, scientists have been investigating the benefits of napping, including the 30-minute nap as well as sleep durations of 1–2 hours. Performance across a wide range of cognitive processes has been tested. Studies demonstrate that naps are as good as a night of sleep for some types of memory tasks. A NASA study led by David F. Dinges, professor at the University of Pennsylvania School of Medicine, found that naps can improve certain memory functions and that long naps are more effective than short ones. In that NASA study, volunteers spent several days living on one of 18 different sleep schedules, all in a laboratory setting. To measure the effectiveness of the naps, tests probing memory, alertness, response time, and other cognitive skills were used.
   Section::::Systematic napping as a lifestyle.
  
  A contemporary idea called polyphasic sleeping entails avoiding long periods of sleep, instead taking regularly spaced short naps. Sara Mednick, whose sleep research investigates the effects of napping, included a chapter, "Extreme Napping", in her book "Take a Nap!". In response to questions from readers about the "uberman" schedule of "polyphasic sleeping", she commented as follows:
   These findings also show that the greatest decline in blood pressure occurs between lights-off and onset of daytime sleep itself. During this sleep period, which lasted 9.7 minutes on average, blood pressure decreased, while blood vessel dilation increased by more than 9 percent.
  
  "There is little change in blood pressure once a subject is actually asleep," Zaregarizi noted, and he found minor changes in blood vessel dilation during sleep (Zaregarizi, M. 2007 & 2012).
  
  Section::::Negative effects.
  
  For those suffering from insomnia or depression, naps may aggravate already disrupted sleep-wake patterns.
  
  Section::::Power nap.
   Sara Mednick conducted a study experimenting on the effects of napping, caffeine, and a placebo. Her results showed that a 60–90-minute nap is more effective than caffeine in memory and cognition.
  
  Section::::Stimulant nap.
   Experimental confirmation of the benefits of this brief nap comes from a Flinders University study in Australia in which 5, 10, 20, or 30-minute periods of sleep were given. The greatest immediate improvement in measures of alertness and cognitive performance came after the 10 minutes of sleep. The 20 and 30-minute periods of sleep showed evidence of sleep inertia immediately after the naps and improvements in alertness more than 30 minutes later but not to a greater level than after the 10 minutes of sleep.
   Nap
  
  A nap is a short period of sleep, typically taken during daytime hours as an adjunct to the usual nocturnal sleep period. Naps are most often taken as a response to drowsiness during waking hours. A nap is a form of biphasic or polyphasic sleep, where the latter terms also include longer periods of sleep in addition to one single period.
   Zaregarizi and his team have concluded that the acute time of falling asleep was where beneficial cardiovascular changes take place. This study has indicated that a large decline in blood pressure occurs during the daytime sleep-onset period only when sleep is expected; however, when subjects rest in a supine position, the same reduction in blood pressure is not observed. This blood pressure reduction may be associated with the lower coronary mortality rates seen in the Mediterranean and Latin American populations where siestas are common. Zaregarizi assessed cardiovascular function (blood pressure, heart rate, and measurements of blood vessel dilation) while nine healthy volunteers, 34 years of age on average, spent an hour standing quietly; reclining at rest but not sleeping; or reclining to nap. All participants were restricted to 4 hours of sleep on the night prior to each of the sleep laboratory tests. During three daytime naps, he noted significant reductions in blood pressure and heart rate. By contrast, the team did not observe changes in cardiovascular function while the participants were standing or reclining at rest.
   Claudio Stampi, as a result of his interest in long-distance solo boat racing, has studied the systematic timing of short naps as a means of ensuring optimal performance in situations where extreme sleep deprivation is inevitable, but he does not advocate ultrashort napping as a lifestyle. "Scientific American Frontiers" (PBS) has reported on Stampi's 49-day experiment where a young man napped for a total of three hours per day. It purportedly shows that all stages of sleep were included. Stampi has written about his research in his book "" (1992). In 1989 he published results of a field study in the journal "Work & Stress", concluding that "polyphasic sleep strategies improve prolonged sustained performance" under continuous work situations. In addition, other long-distance solo sailors have documented their techniques for maximizing wake time on the open seas. One account documents the process by which a solo sailor broke his sleep into between 6 and 7 naps per day. The naps would not be placed equiphasically, instead occurring more densely during night hours.
   A NASA study led by David F. Dinges, professor at the University of Pennsylvania School of Medicine, found that naps can improve certain memory functions. In that NASA study, volunteers spent several days living on one of 18 different sleep schedules, all in a laboratory setting. To measure the effectiveness of the naps, tests probing memory, alertness, response time, and other cognitive skills were used.
  
  Power Napping Enablers and sleep timers allow properly timed power napping.
   Excessive sleepiness can vary in severity, and it appears most commonly during monotonous situations that doesn’t require much interaction. Daytime naps may occur with little warning and may be physically irresistible. These naps can occur several times a day. They are typically refreshing, but only for a few hours or less. Vivid dreams may be experienced on a regular basis, even during very brief naps. Drowsiness may persist for prolonged periods or remain constant. In addition, night-time sleep may be fragmented, with frequent awakenings. A second prominent symptom of narcolepsy is abnormal REM sleep. Narcoleptics are unique in that they enter into the REM phase of sleep in the beginnings of sleep, even when sleeping during the day.
   Understanding the activity of different parts of the brain during sleep can give a clue to the functions of sleep. It has been observed that mental activity is present during all stages of sleep, though from different regions in the brain. So, contrary to popular understanding, the brain never completely shuts down during sleep. Also, sleep intensity of a particular region is homeostatically related to the corresponding amount of activity before sleeping. The use of imaging modalities like PET and fMRI, combined with EEG recordings, gives a clue to which brain regions participate in creating the characteristic wave signals and what their functions might be.
   This two process model was first proposed in 1982 by Borbely, who called them Process S (homeostatic) and Process C (Circadian) respectively. He showed how the slow wave density increases through the night and then drops off at the beginning of the day while the circadian rhythm is like a sinusoid. He proposed that the pressure to sleep was the maximum when the difference between the two was highest.
   Section::::EEG and sleep.
   Sleep and metabolism
  
  Sleep is important in regulating metabolism. Mammalian sleep can be sub-divided into two distinct phases - REM (rapid eye movement) and non-REM (NREM) sleep. In humans and cats, NREM sleep has four stages, where the third and fourth stages are considered slow-wave sleep (SWS). SWS is considered deep sleep, when metabolism is least active.
   In terms of actual numbers, an infant from one to three months of age may sleep sixteen to eighteen hours a day in periods that last from three to four hours. By three months the period of sleep lengthens to about four or five hours, with a decrease in the total sleep time to about fourteen or fifteen hours. At three months, they also start to sleep when it is dark and wake when it is light. By four months there are two distinct napping periods: mid-morning and late afternoon. By six months the longest LSP is six hours and occurs during the night. There are two three-or-more hour naps with a total average sleep time of fourteen hours.
   People who regularly take these short naps, or catnaps, may develop a good idea of the duration which works best for them, as well as which tools, environment, position, and associated factors help produce the best results. Power naps are effective even when schedules allow a full night's sleep. Mitsuo Hayashi and Tadao Hori have demonstrated that a nap improves mental performance, even after a full night's sleep.
  
  Section::::Stimulant or caffeine nap.
   "There is something very specific about the timing of the nap," she is quoted as saying in "The Times" (London). "It should be at about 2pm or 3pm. It's the time when most humans and animals experience what is called a post-prandial dip or low ebb. It's a dip in cogno-processing and physiological responses, when a lot of us actually do feel sleepy."
   A short nap preceded by the intake of caffeine was investigated by British researchers. In a driving simulator and a series of studies, Horne and Reyner looked at the effects of cold air, radio, a break with no nap, a nap, caffeine pill vs. placebo and a short nap preceded by caffeine on mildly sleep-deprived subjects. The caffeine nap was by far the most effective in reducing driving "incidents" and subjective sleepiness. Caffeine in coffee takes up to a half-hour to have an alerting effect, hence "a short (15min) nap will not be compromised if it is taken immediately after the coffee."
   A Flinders University study of individuals restricted to only five hours of sleep per night found a 10-minute nap was overall the most recuperative nap duration of various nap lengths they examined (lengths of 0 min, 5 min, 10 min, 20 min, and 30 minutes): the 5-minute nap produced few benefits in comparison with the no-nap control; the 10-minute nap produced immediate improvements in all outcome measures (including sleep latency, subjective sleepiness, fatigue, vigor, and cognitive performance), with some of these benefits maintained for as long as 155 minutes; the 20-minute nap was associated with improvements emerging 35 minutes after napping and lasting up to 125 minutes after napping; and the 30-minute nap produced a period of impaired alertness and performance immediately after napping, indicative of sleep inertia, followed by improvements lasting up to 155 minutes after the nap.
  