Re: What is the genetic mechanism behind sleep ?

Sleep is a big topic. To get started, check out this review that spans fruit flies to humans to get an idea of genetic similarities.
http://www.ncbi.nlm.nih.gov/pubmed/21784243

Once you get an idea of the genetics of sleep, look at the complicated neurophysiology here:
http://www.ncbi.nlm.nih.gov/pubmed/22811426

The biggest question you pose is the first -- why we sleep when we do. Is it because we are 'tired'. Unclear on the meaning -- physical exhaustion or some mental matter -- studying too long, too hard. But what about getting sleepy in front of the television? No real work there, not even brain work (or much, anyway). But some people find it easier to fall asleep with the tv on rather than off (not me, it is a distraction). I would guess that we sleep because we need to. We feel tired when we need the sleep and whatever sleep deficit accumulator has reached a particular amount. This means that we feel tired when we receive the signal to sleep. (See the first reference.)

The most people will venture to conjecture about sleep is that we optimally function when we sleep at prescribed intervals for prescribed period of time (individually varying, of course). It seems that we have endogenous metabolic (meaning biochemical) rhythms not only of neurotransmitter and nutrient turnover but also oscillations of stress and appetitive hormones. These are inherent, meaning a result of the genetic transcription control networks. They become 'entrained' to the diurnal or day-night cycle by at least one known input of retinal ganglion cell axons to the suprachiasmatic nucleus, which seems to be able to phase the metabolic cycle with the photocycle. It is curious because people living all the time in caves, where they receive no daylight, have 24 to 25 (sometimes 30) hr period metabolic (e.g., cortisol, ACTH) cycles that gradually become out of phase with daylight.