Maybe you're not tired because you didn’t get enough sleep or because you worked too hard.
Maybe you’re tired because you didn’t plan out your day and you don’t have a clear purpose for the day. Or maybe you’re tired because the work you’re doing isn’t exciting to you.
Find a purpose in your work, something that transcends the work itself, and find direction on a daily basis to keep yourself engaged and full of energy.
Once the direction is gone, it’s time to go to sleep.