Everybody has their own internal clock and individuals have various sleep needs. Nonetheless, there are certain factors which affect how many hours of sleep you need.
For instance, sleep quality, age, exercise, stress, environmental conditions, and pregnancy. But overall, age is the greatest factor. Below are some general guidelines from the National Sleep Foundation, recommending the amount of sleep each age group needs.

The REM stage of sleep is unique because it’s when you have your most vivid dreams. During your first cycle of sleep, it starts about 90 minutes after you fall asleep and last only 10 minutes. Each cycle after that, your REM sleep gets longer and longer. In the final cycle is may last up to an hour.

During your REM sleep cycle, your eyes are darting around and “seeing” different things. Your brain is also converting experiences into memories. Furthermore, it enhances your ability for creative problem-solving. These are all important reasons to make sure your alarm doesn’t wake you up during this crucial stage of sleep.

To put it simply, a sleep cycle is what your brain does while you’re getting your zzz’s. As you sleep, your brain goes through various patterns of activity. These patterns are based on distinct eye movements and muscle activity. In fact, sleep scientists have tracked these movements and found two main sleep cycle categories: rapid-eye-movement (REM) sleep and non-rapid-eye-movement (NREM) sleep.

Your natural sleep cycle lasts about 90 minutes. During this time your brain moves through five different stages.

A typical night’s sleep consists of about 5-6 full sleep cycles. But each of these sleep cycles is different from one another. During the first 2-3 sleep cycles, you spend most of the time in a deep NREM sleep. In the final 2-3 sleep cycles, you will spend more time in REM sleep and stage one light sleep. By using a sleep cycle calculator (such as the one here), you can time your alarm to go off during your lightest sleeping period.