Lot's of good responses but there's one thing I'm missing: working out.
A few years ago I was playing icehockey maybe once a day or something and I felt great, school was easy and I had a lot of energy to spare (well, for a teenager anway).
Then I quit hockey and I started doing nothing, just sitting in front of the computer and guess what happened? I got inexplicitly tired and burned out. I love math but it got boring and my arms where like spaghetti and I almost tanked school (thankfully I didn't). I've always thought it was lack of challenges but now I know it wasn't that simple.
I think that my lack of working out really did make me more tired, and not just a bit but extremely tired and depressed and a whole heap of things. Now when I've started at the university I had this same strange sensation like back then, much milder but still familiar. So I went to a couple of taekwondo practices and I'm feeling a ton better already.
So my suggestion is to start a regular, and then I do mean regular with no skipping because you don't feel for it or don't have time etc, workout. You don't have to go for a run if you don't think that's the most fun thing there is (I personally really dislike it) but find something fun - ball sports, climbing, dancing whatever.
Besides just discipline and energy, I also noticed one more thing working out helped me out with.
It provides a way to push your limits without being afraid of failure - because there is no pressure to perform. You can't lift the weight, no worries, just drop down a plate this week and give it a go the following week! This attitude propagates to work - you tend to take away failure as an excuse not to get something done!
I've made working out a habit for a short while now and it really has done a lot for my attitude and energy levels. If you aren't already doing some regular physical activity I also recommend making a hobby out of something that keeps you moving.
On the other hand, if you hate working out and working, and you have a limited amount of willpower, you might end up spending all of your willpower working out.
True. But you're probably exerting yourself too hard if a workout session taxes you that much (or you have it built up in your mind so you'll hate it no matter what). Remember Worf's exercise of holding 13-pound weights straight in front of him? To do that, you don't even start with weights, you just hold your arms out. There's no shame (literally none, this is actually the secret of developing an exercise mentality/physiology) in lifting incredibly light things, even just your arms.
I do hear you, willpower is precious. But since exercise helps create willpower, too, and stress mitigation, it's a highly lucrative investment.
A lot of people just hate things associated with exercise, like going to the gym with a bunch of people, or running around their neighborhood. You can always start by doing bodyweight exercises like pushups, pullups, crunches, and yoga. If you still hate it, you could do what many Americans working menial jobs do and get high beforehand.
A few years ago I was playing icehockey maybe once a day or something and I felt great, school was easy and I had a lot of energy to spare (well, for a teenager anway).
Then I quit hockey and I started doing nothing, just sitting in front of the computer and guess what happened? I got inexplicitly tired and burned out. I love math but it got boring and my arms where like spaghetti and I almost tanked school (thankfully I didn't). I've always thought it was lack of challenges but now I know it wasn't that simple.
I think that my lack of working out really did make me more tired, and not just a bit but extremely tired and depressed and a whole heap of things. Now when I've started at the university I had this same strange sensation like back then, much milder but still familiar. So I went to a couple of taekwondo practices and I'm feeling a ton better already.
So my suggestion is to start a regular, and then I do mean regular with no skipping because you don't feel for it or don't have time etc, workout. You don't have to go for a run if you don't think that's the most fun thing there is (I personally really dislike it) but find something fun - ball sports, climbing, dancing whatever.
Definitely worth a try!