question: Why do you tend go to sleep later at night when you’ve taken a nap in the day? How does your body know that you’ve taken a nap and why does it take it into account?
comment: Think of an hour glass that slowly fills with sand. Your body has chemicals that act the same way, slowly building up within your body...akin to sand slowly building up in an hour class. As more "sand" builds up, the more tired you become. One way to push some "sand" back or get rid of some of the buildup is to sleep. It basically up ends the hour glass for a little bit. A nap will only upend the hour glass for a small amount of time and wont completely reset it. Your body does this naturally with chemicals that slowly build the longer you are awake. Sleeping stops additional buildup and will slowly reverse that chemical buildup.
passages: Section::::Background.
  
  The individual's or animal's circadian phase can be known only by the monitoring of some kind of output of the circadian system, the internal "body clock". The researcher can precisely determine, for example, the daily cycles of gene-activity, body temperature, blood pressure, hormone secretion and/or sleep and activity/alertness. Alertness in humans can be determined by many kinds of verbal and non-verbal tests; activity in animals by observation, for example of wheel-running in rodents.
   An organism whose circadian clock exhibits a regular rhythm corresponding to outside signals is said to be "entrained"; an entrained rhythm persists even if the outside signals suddenly disappear. If an entrained human is isolated in a bunker with constant light or darkness, he or she will continue to experience rhythmic increases and decreases of body temperature and melatonin, on a period which slightly exceeds 24 hours. Scientists refer to such conditions as free-running of the circadian rhythm. Under natural conditions, light signals regularly adjust this period downward, so that it corresponds better with the exact 24 hours of an Earth day.
   Melatonin synthesis is also regulated by the nervous system. Nerve fibers in the retinohypothalamic tract connect the retina to the suprachiasmatic nucleus (SCN). The SCN stimulates the release of Norepinephrine from sympathetic nerve fibers from the superior cervical ganglia that synapse with the pinealocytes. Norepinephrine causes the production of melatonin in the pinealocytes by stimulating the production of cAMP. Because the release of norepinephrine from the nerve fibers occurs at night, this system of regulation maintains the body’s circadian rhythms.
  
  Section::::Melatonin.:Synthesis.
   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.
   The secretion of many hormones is affected by sleep-wake cycles. For example, melatonin, a hormonal timekeeper, is considered a strongly circadian hormone, whose secretion increases at dim light and peaks during nocturnal sleep, diminishing with bright light to the eyes. In some organisms melatonin secretion depends on sleep, but in humans it is independent of sleep and depends only on light level. Of course, in humans as well as other animals, such a hormone may facilitate coordination of sleep onset. Similarly, cortisol and thyroid stimulating hormone (TSH) are strongly circadian and diurnal hormones, mostly independent of sleep. In contrast, other hormones like growth hormone (GH) & prolactin are critically sleep-dependent, and are suppressed in the absence of sleep. GH has maximum increase during SWS while prolactin is secreted early after sleep onset and rises through the night. In some hormones whose secretion is controlled by light level, sleep seems to increase secretion. Almost in all cases, sleep deprivation has detrimental effects. For example, cortisol, which is essential for metabolism (it is so important that animals can die within a week of its deficiency) and affects the ability to withstand noxious stimuli, is increased by waking and during REM sleep. Similarly, TSH increases during nocturnal sleep and decreases with prolonged periods of reduced sleep, but increases during total acute sleep deprivation. 
   The earliest recorded account of a circadian process dates from the 4th century BC, when Androsthenes, a ship captain serving under Alexander the Great, described diurnal leaf movements of the tamarind tree. The observation of a circadian or diurnal process in humans is mentioned in Chinese medical texts dated to around the 13th century, including the "Noon and Midnight Manual" and the "Mnemonic Rhyme to Aid in the Selection of Acu-points According to the Diurnal Cycle, the Day of the Month and the Season of the Year".
   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.
   Circadian clock
  
  A circadian clock, or circadian oscillator, is a biochemical oscillator that cycles with a stable phase and is synchronized with solar time.
  
  Such a clock's "in vivo" period is necessarily almost exactly 24 hours (the earth's current solar day). In most living things, internally synchronized circadian clocks make it possible for the organism to anticipate daily environmental changes corresponding with the day–night cycle and adjust its biology and behavior accordingly.
   Melatonin is absent from the system or undetectably low during daytime. Its onset in dim light, "dim-light melatonin onset" (DLMO), at roughly 21:00 (9 p.m.) can be measured in the blood or the saliva. Its major metabolite can also be measured in morning urine. Both DLMO and the midpoint (in time) of the presence of the hormone in the blood or saliva have been used as circadian markers. However, newer research indicates that the melatonin "offset" may be the more reliable marker. Benloucif et al. found that melatonin phase markers were more stable and more highly correlated with the timing of sleep than the core temperature minimum. They found that both sleep offset and melatonin offset are more strongly correlated with phase markers than the onset of sleep. In addition, the declining phase of the melatonin levels is more reliable and stable than the termination of melatonin synthesis.
   Weather events influence biological processes on short time scales. For instance, as the Sun rises above the horizon in the morning, light levels become sufficient for the process of photosynthesis to take place in plant leaves. Later on, during the day, air temperature and humidity may induce the partial or total closure of the stomata, a typical response of many plants to limit the loss of water through transpiration. More generally, the daily evolution of meteorological variables controls the circadian rhythm of plants and animals alike.
   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.
   The circadian clock exerts constant influence on the body, effecting sinusoidal oscillation of body temperature between roughly 36.2 °C and 37.2 °C. The suprachiasmatic nucleus itself shows conspicuous oscillation activity, which intensifies during subjective day (i.e., the part of the rhythm corresponding with daytime, whether accurately or not) and drops to almost nothing during subjective night. The circadian pacemaker in the suprachiasmatic nucleus has a direct neural connection to the pineal gland, which releases the hormone melatonin at night. Cortisol levels typically rise throughout the night, peak in the awakening hours, and diminish during the day. Circadian prolactin secretion begins in the late afternoon, especially in women, and is subsequently augmented by sleep-induced secretion, to peak in the middle of the night. Circadian rhythm exerts some influence on the nighttime secretion of growth hormone.
   According to the Hobson & McCarley activation-synthesis hypothesis, proposed in 1975–1977, the alternation between REM and non-REM can be explained in terms of cycling, reciprocally influential neurotransmitter systems. Sleep timing is controlled by the circadian clock, and in humans, to some extent by willed behavior. The term circadian comes from the Latin "circa", meaning "around" (or "approximately"), and "diem" or dies, meaning "day". The circadian clock refers to a biological mechanism that governs multiple biological processes causing them to display an endogenous, entrainable oscillation of about 24 hours. These rhythms have been widely observed in plants, animals, fungi and cyanobacteria.
   Light is one of the most defining environmental factors that determines an animal’s activity pattern. Photoperiod or a light dark cycle is determined by the geographical location, with day time being associated with lots of ambient light, and night time being associated with little ambient light. Light is one of the strongest influences of the suprachiasmatic nucleus (SCN) which is part of the hypothalamus in the brain that controls the circadian rhythm in most animals. This is what determines whether an animal is diurnal or not. The SCN uses visual information like light to start a cascade of hormones that are released and work on many physiological and behavioural functions.
   Nocturnal animals have higher body temperatures, greater activity, rising serotonin, and diminishing cortisol during the night—the inverse of diurnal animals. Nocturnal and diurnal animals "both" have increased electrical activity in the suprachiasmatic nucleus, and corresponding secretion of melatonin from the pineal gland, at night. Nocturnal mammals, which tend to stay awake at night, have higher melatonin at night just like diurnal mammals do. And, although removing the pineal gland in many animals abolishes melatonin rhythms, it does not stop circadian rhythms altogether—though it may alter them and weaken their responsiveness to light cues. Cortisol levels in diurnal animals typically rise throughout the night, peak in the awakening hours, and diminish during the day. In diurnal animals, sleepiness increases during the night.
   Section::::In mammals.:Biological markers and effects.
  
  The classic phase markers for measuring the timing of a mammal's circadian rhythm are:
  
  BULLET::::- melatonin secretion by the pineal gland,
  
  BULLET::::- core body temperature minimum, and
  
  BULLET::::- plasma level of cortisol.
   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.
   Humans, like most living organisms, have various biological rhythms. These biological clocks control processes that fluctuate daily (e.g. body temperature, alertness, hormone secretion), generating circadian rhythms. Among these physiological characteristics, our sleep-wake propensity can also be considered one of the daily rhythms regulated by the biological clock system. Our sleeping cycles are tightly regulated by a series of circadian processes working in tandem, which allow us to experience moments of consolidated sleep during the night and a long wakeful moment during the day. Conversely, disruptions to these processes and the communication pathways between them can lead to problems in sleeping patterns, which are collectively referred to as Circadian rhythm sleep disorders.
   The first recorded observation of an endogenous circadian oscillation was by the French scientist Jean-Jacques d'Ortous de Mairan in 1729. He noted that 24-hour patterns in the movement of the leaves of the plant "Mimosa pudica" continued even when the plants were kept in constant darkness, in the first experiment to attempt to distinguish an endogenous clock from responses to daily stimuli.
   The WELL Building standard additionally provides direction for circadian emulation in multi-family residences. In order to more accurately replicate natural cycles lighting users must be able to set a wake and bed time. A equivalent melanopic lux of 250 must be maintained in the period of the day between the indicated wake time and two hours before the indicated bed time. An equivalent melanopic lux of 50 or less is required for the period of the day spanning from two hours before the indicated bed time through the wake time. In addition at the indicated wake time melanopic lux should increase from 0 to 250 over the course of at least 15 minutes.
   In humans and animals, there is a regulatory system that governs the phase relationship of an organism's internal circadian clock to a regular periodicity in the external environment (usually governed by the solar day). In most organisms, a stable phase relationship is desired, though in some cases the desired phase will vary by season, especially among mammals with seasonal mating habits.
   The hormones cortisol and melatonin are effected by the signals light sends through the body's nervous system. These hormones help regulate blood sugar to give the body the appropriate amount of energy that is required throughout the day. Cortisol is levels are high upon waking and gradual decrease over the course of the day, melatonin levels are high when the body is entering and exiting a sleeping status and are very low over the course of waking hours. The earth's natural light-dark cycle is the basis for the release of these hormones.
   Sleep timing depends greatly on hormonal signals from the circadian clock, or Process C, a complex neurochemical system which uses signals from an organism's environment to recreate an internal day–night rhythm. Process C counteracts the homeostatic drive for sleep during the day (in diurnal animals) and augments it at night. The suprachiasmatic nucleus (SCN), a brain area directly above the optic chiasm, is presently considered the most important nexus for this process; however, secondary clock systems have been found throughout the body.
   Sleep regulation refers to the control of when an organism transitions between sleep and wakefulness. The key questions here are to identify which parts of the brain are involved in sleep onset and what their mechanisms of action are. In humans and most animals sleep and wakefulness seems to follow an electronic flip-flop model i.e. both states are stable, but the intermediate states are not. Of course, unlike in the flip-flop, in the case of sleep, there seems to be a timer ticking away from the minute of waking so that after a certain period one must sleep, and in such a case even waking becomes an unstable state. The reverse may also be true to a lesser extent.
   In a study done on the effect of lighting intensity on delta waves, a measure of sleepiness, high levels of lighting (1700 lux) showed lower levels of delta waves measured through an EEG than low levels of lighting (450 lux). This shows that lighting intensity is directly correlated with alertness in an office environment.
  