In that case, there's nothing you can really do to feel less tired after a day in the sun besides getting some rest — and trying not to spend so much time in the sun the next day. If you're concerned about feeling knocked out all the time (not just after a day at the beach) no matter how much rest you get, that's worth checking in with a doctor about. It could be a sign of a bigger issue, such as a sleep disorder . At the end of the day, this is just another reason to be smart about your sun exposure, and remember that you don't have to spend a ton of time outside if you're more comfortable resting somewhere cool and dark — like, you know, your bed.