A new Belgian study shows that people rely on two different internal “time keeping devices” to regulate their sleep patterns. This study could, in the future, be able to help those who are suffering from jet lag or help those who work the third shift, or graveyard shift, return to normal sleep patterns during their time off.
The results come at the heels of a study from the University of Liege in which 33 participants volunteered to stay up for 42 hours straight. During this time, their mental function was tested with performance tests and those requiring reaction times. Not surprisingly, the longer the individual remained awake, the worse his or her performance became.
During the study, the participants also had MRI scans done to look at brain wave interaction. What was revealed was a complex interaction, and maybe even competition, between the brain’s circadian rhythm and homeostatic sleep drive.
The circadian rhythm exists in order to keep people awake during daylight hours and make them sleepy when the sun goes down, while the homeostatic sleep drive kicks in when someone has been awake too long, regardless of the time of day. Because of this, researchers stated that the homeostatic sleep drive was more like an hourglass as compared to the circadian rhythm’s clock. 
Dr. Charles Czeisler, a professor of sleep medicine at Harvard Medical School in Boston said of human sleep patterns:
“The primary determinant of how long you sleep is not the amount of time you’ve been awake. It’s what ‘time’ it is in your body.”
This is why when you’ve been awake for an excessively-long period due to traveling or other stressful events, you don’t sleep for several hours to make up for it. Instead, you may fall asleep for a few hours, but your body will wake you up again because of the time your body perceives it as.
It is recommended that those under 65 get 7 to 9 hours of sleep per night, while those over 65 get at least 8 hours per night. Czeisler, however, says that the amount of sleep required will be different for each person, thus it is important to pay attention to bodily clues.