I don't sleep at all during the night but I know why. This is a occurring problem especially in gamers. You'll be playing a game, or on your phone or watching your movie etc and then you get tired and want to sleep. When you put down your phone to go to sleep, it'll be much harder to because your brain hasn't built up enough melatonin due to the fact you had lights infront of your eyes for numerous hours before bed time.
Many many years ago, when populated areas weren't as bright as they are today, melatonin would have been slowly building throughout the day at various times, in modern day that doesn't happen as much. Seems like theres always light around, and sadly being exposed to light blocks the production of Melatonin.
That is generally why your body will be tired or exhausted but your mind is wide awake. Or when you have a restless night where you can't stop tossing and turning.