When the sun goes down, experts have noticed that your circadian clock readies your body for bed around two hours before you go to sleep. The more scientific name of circadian rhythm is actually the suprachiasmatic nucleus (SCN). This is a cluster of cells in your brain, directly connected to your retina that wakes you up or makes you drowsy depending on the amount of light in the room. That is also why you may have a difficult time falling asleep when the lights are left on at night.