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.
area of the brain that regulates sleep-wake cycles
hypothalamus