We all have a mechanism that is called the biological clock. This controls the physiological activities that change in any regular cycle. For example, if Justin has to wake up at 7 AM every school day, his body probably wakes him up at the same time in the weekends or holidays.
The reason why Justin wakes up tired is that the alarm clock probably wakes him up before he completes the REM sleep. Every sleep cycle is about 90 minutes. If he sleeps and wakes up naturally, he would complete the stages/cycles of sleep, and feel refreshed afterwards.
Here are the things he could do:
1. Count back 5 cycles of 90 minutes: If he has to wake up at 7 AM it is: 7 > 5:30 > 4 > 2:30 > 1 > 11:30. If he sleeps approximately at 11:30, his cycle will be completed at 7 AM and this will help him feel refreshed after the night.
2. Try to go to bed earlier than usual: By doing this, he can adapt his biological clock and wake up earlier than the alarm clock.
3. Set the alarm clock 10 minutes later than usual: If he can manage this and wake up earlier than the alarm clock, he will only need the alarm clock as a backup not to be late for school.