Sleep architecture describes the structure and pattern of sleep

Sleep

Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and is more easily reversible than...

 and encompasses several variables. Sleep quotas refer to the amount of time spent in REM and NREM sleep. Sleep duration is the total time spent asleep in a 24 hour period. The duration of a NREM-REM cycle (from the beginning of NREM to the end of REM) is also an important aspect of sleep architecture. Phasing of sleep describes how sleep is distributed across a daily cycle: polyphasic sleep

Polyphasic sleep

Polyphasic sleep, a term coined by early 20th-century psychologist J.S. Szymanski, refers to the practice of sleeping multiple times in a 24-hour period—usually more than two, in contrast to biphasic sleep or monophasic sleep . It does not imply any particular sleep schedule...

 consists of multiple bouts of sleep in a 24 hour period, while monophasic sleep consists of a single bout of sleep in a 24 hour period. Sleep architecture varies widely across species, and is thought to be significantly influenced by genetics. The evolution of different types of sleep architecture is influenced by a number of selective pressures

Natural selection

Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....

, including body size, relative metabolic rate, predation, type and location of food sources, and immune function.