Chronotype, deep sleep, and the 3pm crash.
It is real. About 40 to 50 percent of whether you are a short or long sleeper, and whether you are a morning or night person, is written in your genes.
Around half of all adults are slow caffeine metabolisers, and for them the CYP1A2 gene keeps caffeine active in the body for up to ten hours instead of four.
Because time in bed is not the same as deep sleep - variants in PER3 and CLOCK can compress the restorative phases, so eight hours of sleep delivers five hours of recovery.
Not bad - just genetic. A late chronotype is a real, inherited setting, and forcing an early schedule onto it fights your own hormones rather than fixing anything.
Waking at 3am most nights is rarely random and almost never a sign that something is seriously wrong - for many people it is a normal feature of how sleep is built, worsened by a few everyday habits and shaped at the edges by body-clock genes like PER3.
Persistent tiredness despite enough sleep is extremely common, and in the large majority of cases it is not a sign of anything sinister but a stack of ordinary, correctable factors, and the reassuring part is that most of the common causes are findable and fixable once you know where to look.