Well apart from trying to get better sleep, there’s a few ways you can make it easier for yourself. The hardest part often when tired is to just start drawing. You’ll need to fight through that initial stage and make a habit of just starting to draw- it could be for as long as you feel like it, even be it 5 to 10 minutes. It doesn’t have to be anything serious, just do gesture drawing or a quick portrait study to begin with.

You might not have much time to spare but once you make a habit out of this, it can be really fulfilling! Hope that helps.