Sleep!
That’s it. Sleep.
Sleep is one of the best things you can do to boost your immune system. Get enough sleep at night and if you are tired, take a NAP. Naps, or even some down time with a book, are a great way to maximize your energy. I feel like I am stating the obvious, but there you have it.
Insomnia
If you are having trouble sleeping, it is worth following up on that and finding what helps you. Some people find meditating helpful, or getting off/limiting media and screens, particularly before bedtime. Others notice a difference with a diet change, and for many herbal medicine can help.
If you are not getting enough sleep at night, you really deserve to give yourself naptime in the daytime to make up for lost hours.