Your internal 24-hour sleep-wake cycle, otherwise known as biological clock or circadian rhythm, is regulated by processes in the brain that respond to how long you’ve been awake and the changes between light and dark. At night, your body responds to the loss of daylight by producing melatonin, a hormone that makes us sleepy. During the day, sunlight triggers the brain to inhibit melatonin production so you feel awake and alert.
Therefore, you must have experienced it yourself that when you delay your sleep by 2 or 3 hours and go to bed after 1 am or 2 am. You get up 8 or 9′ o clock in the morning you don’t feel fresh and active like others who went to bed by 10 pm and got up at 5 or 6’o clock in the morning. This is simply because you do not tend to get sound sleep during the morning time as atmospheric noise begins to enter your ear and disrupt your sleep.
During the night, your sleep follows a predictable pattern, moving back and forth between deep restorative sleep (deep sleep) and more alert stages (non-REM) and dreaming (REM sleep). Together, the stages of REM and non-REM sleep form a complete sleep cycle. Each cycle typically lasts about 90 minutes and repeats four to six times over the course of a night. This is why if you are sensitive to waking up in the middle of the night, it is probably in the early morning hours, not immediately after going to bed.