How important is sleep for training results?
More important than almost any training variable people obsess over. Short sleep reduces strength and power output, increases perceived effort so sessions feel harder, impairs recovery and muscle protein synthesis, and shifts appetite hormones in a direction that makes eating well harder. Studies restricting sleep to five hours show measurable performance drops within days. Practically: if you are choosing between an extra hour of training and an extra hour of sleep while chronically under-slept, sleep wins. It is also the cheapest and least glamorous performance intervention available.
Answers
The appetite hormone effect is why under-slept weeks are also the weeks my eating falls apart.
Sessions feeling harder on poor sleep is measurable, not imagined. That was reassuring to learn.