Maybe a vacation or sabbatical?
Or if you have spare time and some other type of work piques your interest work some on a different project. I've worked in groups where when it's a down cycle, it's really really down... like boring me to tears down. So I started trying to learn stuff that I never deal with at work, even when it is busy. When I would go back to the regular gig it was nice to look at the same code with a "new set of glasses".
I've had the same issue. It's pretty simple and you identified it. You're working too much. Cut back on hours and eventually your body will refresh and you'll feel energized again.
Get sleep. Get exercise. Eat well. Only drink alcohol on weekends and never during major development stretches.