The Second Wind.
Lets talk about our "second wind" of energy that we get in the evenings. You know, when you are super tired at 10:30 pm and are falling asleep on the coach, then someone puts in an exciting movie and suddenly you are wide-awake and are able to stay up watching the movie with no problem for the next 2+ hours. Or when you are sleepy and ready for bed around 11 pm, but then you start working on a project and become totally motivated, causing you to stay awake until 2 am working on.
What gives us this sudden burst of energy? The hormone cortisol. Cortisol rises and falls during the day, regulating our energy cycles (aka the circadian rhythm). For instance, cortisol peaks in the morning, waking us up in the morning and giving us lots of early daytime energy.
Cortisol is at its lowest before bedtime, which is why you may feel sleepy and yawn a lot. However, if you don't get into bed BEFORE 11 pm (the lowest cortisol/energy level is between 10 and 11 pm), the body semi-panics ("I am still awake, but have no energy!") and as a result releases an additional burst of cortisol. This burst of energy then keeps us awake for awhile longer ...even though we were once really tired. The second wind is a cool phenomenon, but it not a healthy one. Secreting extra cortisol is stressful to the body and pushes it past its limit. By ignoring the initial signs of sleepiness, we are pushing the body past it's natural circadian rhythm, it's natural eb-and-flow of energy. It's ok to tap into your "second wind" every once and awhile, but try to not make this a nightly occurrence.
Also note: As you can see from the image above, having snacks during our natural "low" points increases our energy and our cortisol, preventing the body from entering natural low points. It is important to regularly snack during the day to keep blood sugar levels (and thus perceived energy) from dipping too low.
Image source here.