The key to not waking up feeling tired and groggy is waking up when you are in light sleep. Waking up in the middle of a sleep cycle is going to make you want to hit the snooze button. Sleep cycles last 90 minutes, so if you count back the number of sleep cycles from the time you need to wake up, you arrive at some of the best times for you to hit the sack. Thanks to website sleepyti.me, they handle the calculations for you. Just type in the time you need to wake up and they will present you with the times you should go to bed if you don’t want to wake up feeling like you got hit by a train.