Sleepy Head works by counting backwards in sleep cycles. Sleep cycles typically last 90 minutes. A good night's sleep consists of 5-6 complete sleep cycles. Waking up in the middle of a sleep cycle leaves you feeling tired and groggy, but waking up in between cycles wakes you up feeling refreshed and alert! It takes the average human anywhere in-between 10-30 minutes to fall asleep. For this calculation I've used an absolute average value of 15 minutes.