The inner clock is not the only mechanism regulating your sleep-wake rhythm. Your level of alertness is also affected by something called sleep pressure. Sleep pressure is a chemical indicator telling your body how long you have been awake, and when it is time to get some sleep. Since the moment you first open your eyes in the morning, a neurotransmitter called adenosine starts to accumulate in your brain. As long as you stay awake, your adenosine level keeps rising. By late evening it reaches the threshold, and you will start to feel drowsy. This mechanism is called sleep pressure. The longer you stay awake, the stronger your urge to fall asleep grows.
Sleep pressure also works the other way around. During sleep, your adenosine level decreases, and your urge to sleep starts to ease. By the time your body has given the amount of rest it needs, sleep pressure will reach its baseline level, and you will wake up automatically – that it if you are not forcefully woken up before.
No surprise, you can mess up with your sleep pressure cycles, too. Late-evening naps and accidental dozing off are the worst. Even if you nodded off for just a couple of minutes, your sleep pressure would significantly decrease. Short naps work similarly to caffeine: you will feel more alert than you are. Well-timed power naps can offer a tremendous boost to your productivity after a poorly slept night. Napping is, however, a considerable problem for your sleep if it happens too late. Taking a nap late in the afternoon messes up with the natural rhythm of your sleep pressure. The same goes for dozing off on the couch during the evening. Falling asleep gets much harder, or you might end up waking in the middle of the night.
Example Habits to try:
Borbély, A. A., Daan, S., Wirz-Justice, A., & Deboer, T. (2016). The two-process model of sleep regulation: A reappraisal. Journal of Sleep Research, 25(2), 131–143.