Homeostasis is an essential function, since the organic variables that drift away are driven back to its equilibrium point. For example, if we feel extremely cold, our body will quickly establish mechanisms which tend to restore temperature and, similarly, if we feel extremely hot, it does exactly the same, inversely.
Homeostasis thus drives us to an optimal functioning state and prevents dangerous deviations, since, as it is often said, virtue stands in the middle.
Sleep works as our homeostatic mechanism, coordinating all the little homeostats of the body. For example, if we have a fever, we will sleep more in order to cool the brain; likewise, if we feel cold we will not fall asleep easily.
Apart from this, there is the day and night cycle, a 24-hour variation in light, luminosity, colour and temperature. Borbély’s two-process model did indeed demonstrate that these factors were linked. He simulated the circadian variation with the help of a sinusoidal wave which, just like daylight, cycles continuously between maxima and minima. He simulated homeostasis with an exponential function because, the more we remain awake, the greater our propensity to fall asleep; but, if we sleep a bit, that propensity falls abruptly.
This is what Borbély assumed: there is an exponential rise of sleep propensity, as well as its exponential decrease as soon as sleep occurs, interacting with the regular and sinusoidal circadian oscillation.
Simple as it may seem, it certainly took great skill to reach that conclusion, as well as an enormous perseverance to test it in humans and animals. A giant work carried out by him and his team.