Why can a large meal make you sleepy even when you slept well?

Answer: Signals triggered by eating can reduce the brain’s drive to stay alert

Signals triggered by eating can reduce the brain’s drive to stay alert ✓ — Eating changes signals that help regulate wakefulness, so a large meal can make you feel less alert. Nutrient-sensitive brain circuits and immune signals are among the pathways under study. No single pathway explains every case, and the time of day and meal composition also matter.

Digestion uses up the oxygen that normally keeps the brain awake — Ordinary digestion does not use up the brain’s oxygen supply. The body regulates delivery to the brain while supplying the gut. Post-meal sleepiness involves changes in wakefulness regulation, not the brain running out of oxygen.

The stomach pulls so much blood away that the brain starts shutting down — A normal meal does not make the brain shut down by diverting away its blood. Blood flow to the gut increases during digestion, but that alone does not establish brain starvation. Food-related signals and the daily sleep-wake rhythm offer a better account of ordinary drowsiness.