Some quotations from the following scientific article gives some of the ideas and concepts behind sleep and other cycles.
Introduction
"The relentless cycling of wakefulness and sleep is one of the most pervasive and powerful phenomena experienced by virtually all animals. Initially it was thought that humans and most other mammals spent their lives in two states of being-wakefulness and sleep, with sleep being simply the passive absence of wakefulness(W). It has subsequently become apparent that not only is sleep an active, rather than passive, phenomenon, but that it, in fact, is comprised of two completely different states-rapid eye movement (REM) and nonrapic eye movement (NREM) sleep. Each of these states of being-W/REM/NREM-has its own neuroanatomic, neurophysiologic, neuropharmacologic, and neurochemical substrate, and each sleep state is as different from the other as each is from wakefulness. Furhtermore, each state is comprised of multiple physiologic variables-such as the various tonic and phasic components of REM sleep, which, under normal circumstances, appear simultaneously, resulting in the fully developed state declaration. The cycling of these states is at two levels: the basic wake/sleep cycle (circadian), and the within-sleep cycle of REM and NREM sleep (ultradian; < 24 hr.).
II Overview of Wake/Sleep Cycling
"The cycling of wake and sleep about once every 24 h is traditionally called a circadian rhythm from the latin circa, 'around,' and dies, 'day,' as the rhythm may be aout 1 day. In conditions of temporal isolation when individuals live in experimental bunkers devoid of time cues such as sunlight, television, radios, clock, newspapers, and the like, the period is often close to 25 h, rather than the 24 h cycle of the geophysical day. Many other phenomena also show circadian rhythms, for example, temperature, uninary excretion, and white blood count, to name a few.
"The circadian cycling of human sleep is governed by the suprachiasmatic nucleus (SCN) of the hypothalamus. Although the inherent cycle length for humans is usually longer than 24 h, this cycle is entraned to a 24 h period by the external light/dark cycle.
Implications for the Future
"The full range of cycling of states of being is extremely broad, ranging from yearly (circannual), monthly (circamensual), and daily (circadian). In addition, there are other cyclic phenomena occurring within the day such as cardiac and respiratory rhythms, hormonal fluctuations, and the W/REM/NREM cycles. It is felt that these rhythms are generated independently by poorly undrstood 'pacemakers,' or neuronal/humoral oscillators. Many appear to be entrained by external influences such as the light/dark cycle, and once entrained, may serve to entrain others. It is possible that these cyclical phenomena are kept in regular, periodic cycles by the external pacemakers. Bifurcations in the sleep-wake control system may be associated with a number of facinating and clinically relevant phenomena. The rapid development of a vast array of wake/sleep-REM/NREM monitoring techniques will permit testing this hypothesis."
Mellatonin and sleep - Information
Melatonin and the body's clock
Resetting Circadian Rhythms by Ten Time Zones in Only One Day
Here is an Internet Link on Sleep