<P> Sleep is regulated by two parallel mechanisms, homeostatic regulation and circadian regulation, controlled by the hypothalamus and the suprachiasmatic nucleus (SCN), respectively . Although the exact nature of sleep drive is unknown, homeostatic pressure builds up during wakefulness and this continues until the person goes to sleep . Adenosine is thought to play a critical role in this and many people have proposed that the pressure build - up is partially due to adenosine accumulation . However, some researchers have shown that accumulation alone does not explain this phenomenon completely . The circadian rhythm is a 24 - hour cycle in the body, which has been shown to continue even in the absence of environmental cues . This is caused by projections from the SCN to the brain stem . </P> <P> 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 . </P> <P> In 1993, a different model called the opponent process model was proposed . This model explained that these two processes opposed each other to produce sleep, as against Borbely's model . According to this model, the SCN, which is involved in the circadian rhythm, enhances wakefulness and opposes the homeostatic rhythm . In opposition is the homeostatic rhythm, regulated via a complex multisynaptic pathway in the hypothalamus that acts like a switch and shuts off the arousal system . Both effects together produce a see - saw like effect of sleep and wakefulness . More recently, it has been proposed that both models have some validity to them, while new theories hold that inhibition of NREM sleep by REM could also play a role . In any case, the two process mechanism adds flexibility to the simple circadian rhythm and could have evolved as an adaptive measure . </P> <P> Much of the brain activity in sleep has been attributed to the thalamus and it appears that the thalamus may play a critical role in SWS . The two primary oscillations in slow wave sleep, delta and the slow oscillation, can be generated by both the thalamus and the cortex . However, sleep spindles can only be generated by the thalamus, making its role very important . The thalamic pacemaker hypothesis holds that these oscillations are generated by the thalamus but the synchronization of several groups of thalamic neurons firing simultaneously depends on the thalamic interaction with the cortex . The thalamus also plays a critical role in sleep onset when it changes from tonic to phasic mode, thus acting like a mirror for both central and decentral elements and linking distant parts of the cortex to co-ordinate their activity . </P>

Area of the brain that regulates sleep-wake cycles