Two main biological factors that affect sleep are ‘circadian clock’ & ‘sleep homeostasis’. The second one is a kind of internal timer for sleep. The longer you have been awake, the more you feel the need for sleep. It builds sleep pressure on you throughout the time you are awake & you feel tired & sleepy. When you go to sleep, that sleep pressure releases and a lot of things happen in your body & mind! Your body temperature & blood pressure drops, breathing becomes slower, blood supply to muscles increased, body & muscles are relaxed, many growth hormones are released & energy is restored. All these processes provides energy to your brain & body & make you prepare for your daytime performance. That’s why you feel less tired & more relaxed on wake up!