Is waking up at 5am actually better, or is that just productivity culture?

Mostly culture. Chronotype is substantially genetic, and forcing a natural night owl onto an early schedule tends to produce chronic sleep debt rather than productivity. What the research supports is regularity rather than earliness — a consistent sleep and wake time matters more than the specific hours. The apparent advantage of early risers largely disappears once total sleep is accounted for. Early rising does have a practical benefit for many: fewer interruptions. But going to bed at 1am and rising at 5am is not a routine, it is sleep deprivation with better marketing.