Why Do I Wake Up Tired?
Sleep is not a single, solid block of time. It happens in waves called Sleep Cycles, which typically last about 90 minutes. During a cycle, your brain moves through stages:
- Light Sleep (NREM 1 & 2): Your body relaxes, heart rate slows. Easy to wake up.
- Deep Sleep (NREM 3): Physical restoration happens here. Very hard to wake up; you will feel "sleep drunk" if an alarm goes off now.
- REM (Rapid Eye Movement): Dreaming and mental restoration.
The goal of this calculator is to wake you up at the end of a cycle (when you are in light sleep), rather than in the middle of deep sleep. Waking up at the right moment can make 7.5 hours of sleep feel better than 8.5 hours.
Sleep Inertia Explained
If your alarm screams while you are in Deep Sleep, your brain has to jump from Delta waves (0.5-4 Hz) to Beta waves (12-30 Hz) instantly. This shock causes Sleep Inertia—that heavy, groggy feeling that can last for hours, no matter how much coffee you drink.
Frequently Asked Questions
Is 6 hours (4 cycles) enough? ▼
For most adults, 5 cycles (7.5 hours) is optimal. 4 cycles (6 hours) is generally considered the absolute minimum for long-term health. Sleeping less than 6 hours chronically is linked to higher risks of heart disease and obesity.
What if I wake up before my alarm? ▼
Get up! If you wake up naturally 10 minutes before your alarm, it means your body has finished a sleep cycle. Going back to sleep for those 10 minutes will just start a new cycle that your alarm will interrupt, making you feel worse.