Many factors play a role as to why a person wakes up at a specific time every day/night. Anxiety, bright light, hormone levels, acid reflux, jet lag among many others can affect a person's sleep cycle. A biological clock regulates the bodies timing process and with that comes circadian rhythm that controls a person's sleep cycle, which includes REM sleep and non-REM sleep. These are steps in the sleep cycle, which are important to help stimulate the brain and allow it to rest. According to "The Conversation", there are at least 15 genes within the body that help control the sleep cycle. SIKI, is a gene that revealed a new pathway relating to resetting the a person's internal clock. Another gene discovered gives someone a big advantage especially if they fly to different time zones a lot. ID2 is a gene that that regulates a person's internal clock based on light. ID2 can help with jet lag because the person with this gene is able to adjust to a new time zone faster than others. Overall there are many different explanations that can answer why someone wakes up a specific time every night. Some of which can be controlled by setting a regular sleep schedule and others can't.