Sleep is a naturally recurring state of our mind and body. The main characteristics of sleep are reduced consciousness, inhibited sensory activity, decreased ability to react to stimulus, and inhibition of almost all voluntary muscles (skeletal muscles). Sleep occurs in repeating periods and in two distinct modes: Rapid eye movement sleep (REM sleep) and non-rapid eye movement sleep (non-REM Sleep).
The circadian rhythm is our 24-hour cycle that controls our body temperature, hunger and sleep. Light has a big influence on it. In the morning, when light floods our brain, it tells our brain to stop making melatonin, the sleeping hormone, and later in the day flips the melatonin production back on for us to sleep.