I can get very tired in the late afternoon and early evening. But if I stay up past 8pm, I get revived. A naturopathic doctor explained that it's due to how adrenal glands stop producing and go into a rest state after sunset. But they'll start up again after 10pm or so if you're still awake. Eventually, they get depleted.

There's a TCM Body Clock that supposedly explains why we might awaken at 3am (liver issues) or why we can't wake up refreshed at 7am (exhausted adrenals).