What causes waking up tired after a full night?

Waking up tired after a full night of sleep usually comes down to sleep quality rather than quantity. The most common culprit is sleep apnea, where breathing repeatedly stops and starts, preventing deep restorative sleep. Even without apnea, fragmented sleep from stress, alcohol, or an inconsistent schedule disrupts the natural sleep cycles needed to feel rested. Other frequent causes include sleeping at the wrong circadian phase, meaning your body clock is misaligned with when you actually sleep. Anemia, thyroid disorders, depression, and dehydration also commonly cause morning fatigue. Poor sleep hygiene like screen exposure before bed reduces slow-wave and REM sleep. If it persists, a sleep study can rule out apnea or other disorders.