This Is How Your Body Knows When To Wake Up

Our body does a number of things during the course of the night when we're asleep. While we're catching up on some much-needed rest and recuperation, our brain is busy keeping watch during the night shift. According to, the roughly 16 hours a day we spend awake, and the eight hours we spend asleep at night, make up the 24-hour period known as the body's sleep-wake cycle. The transitional periods during this cycle from sleep to wakefulness and from wakefulness to sleep are triggered by chemical messages in the brain sent by neurotransmitters (via Johns Hopkins Medicine).

For example, two chemicals that prompt feelings of drowsiness when it's time for sleep are melatonin and adenosine (via American Chemical Society). Adenosine levels progressively build up in the blood during the course of the day, leaving us feeling sleepy by the time bedtime rolls around. Additionally, our body releases melatonin as a response to the absence of light. As melatonin continues to be produced throughout the evening and into the night, it causes us to feel sleepy. So what chemicals are responsible for our body's wake response? Even more, how is it that our body seems to know when it's time to wake up even when we don't set an alarm?

Our body wakes us up when prompted by certain triggers

According to TeachMe Physiology, there are two major systems responsible for waking us up when it's time: the reticular activating system (RAS) and the brain area known as the suprachiasmatic nucleus (SCN). Each of these systems responds to either internal and/or external stimuli. The RAS creates chemical messages prompting wakefulness in response to things such as the buzzing of an alarm clock, the need to pee in the middle of the night, or the dreaded "time to wake up!" call from your mom downstairs (via The Conversation).

But what if we're not being directly prompted by anything around us? How does our body know when to wake up when it's the weekend and we have the alarm clock switched off? In these cases, it's the suprachiasmatic nucleus (SCN) that responds to a different kind of trigger: natural light (via Johns Hopkins Medicine). When the optic nerve in your eyes detects the presence of sunlight, the SCN is activated and releases chemicals such as cortisol to transition us into a state of wakefulness. It's the SCN that's also responsible for sending chemical messages prompting the release of melatonin, which conversely, eases us into a state of sleep at night.

So whether you're being woken up by a barking dog or an early morning sunrise, your brain will know when it's time to start the day.