Why 7-9 hours
Sleep is when your body repairs itself and your brain processes information from the day. During sleep, your immune system strengthens, your muscles grow and repair, and your brain forms memories. Getting the recommended 7-9 hours allows these important processes to happen fully. When you don't get enough sleep, these processes get interrupted and your body cannot function at its best.
Individual differences
Not everyone needs exactly 8 hours. Some people are short sleepers and feel great on 6 hours, while others are long sleepers and need 10 hours. Your genetics, age, activity level, and overall health affect how much sleep you personally need. The best approach is to notice how you feel with different amounts of sleep and aim for the amount that makes you feel alert and healthy.
Sleep quality is important
Getting 8 hours of interrupted, poor-quality sleep is not as good as getting 7 hours of deep, uninterrupted sleep. Quality sleep means falling asleep easily, staying asleep through the night, and waking up feeling refreshed. Factors that improve sleep quality include keeping a consistent sleep schedule, making your bedroom dark and cool, and avoiding screens before bed.
Effects of sleep deprivation
Regularly getting less than 7 hours of sleep is linked to serious health problems. These include obesity, type 2 diabetes, high blood pressure, heart disease, stroke, and depression. In the short term, not enough sleep affects your mood, concentration, reaction time, and ability to learn. Even one night of poor sleep can affect how you perform the next day.
Tips for getting enough sleep
To help yourself get 7-9 hours, go to bed and wake up at the same time every day, even on weekends. Keep your bedroom cool, dark, and quiet. Avoid caffeine in the afternoon and evening. Exercise regularly, but not close to bedtime. Put away phones and other screens at least one hour before sleep. If you struggle with sleep, talk to a doctor about what might be causing the problem. 
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