system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it better to get uninterrupted sleep than the same number of hours of fragmented sleep? And why do some people require more sleep than others?

I'm tired, Reddit.
gpt
Not all sleep is the same. You go thru stages over the hours, from light sleep (alpha) to REM. 

During periods of relaxation, while still awake, our brain waves become slower, increase in amplitude and become more synchronous. These types of waves are called alpha waves.

The first stage of sleep is characterized by theta waves. The difference between relaxation and stage 1 sleep is gradual and subtle. As the sleeper moves to stage 2 sleep theta wave activity continues. 

Stages 1 and 2 are relatively "light" stages of sleep. In fact, if someone is awoken during one of these stages, he or she will often report no being asleep at all. That's why fragmented sleep is not good, you don't go beyond stage 2. 

As you continue to shut down your brain patterns change to delta wave patterns. Delta sleep is our deepest sleep, the point when our brain waves are least like waking. 

After that you sometimes get to REM (rapid eye movement). It's theorized that REM helps the brain clean up memories accumulated during the waking periods (dreams). For that reason hormones are released so the skeletal muscles of a person during REM sleep are effectively paralyzed so you don't act out your dreams. 

REM sleep a sleepers brain waves demonstrate characteristics that are similar to waking sleep, a combination of alpha, beta, and desynchronous waves. 

In a normal night's sleep, a sleeper begins in stage 1, moves down through the stages, to stage 4, then back up through the stages, with the exception that stage 1 is replaced by REM, then the sleeper goes back down through the stages again. **One cycle, from stage 1 to REM takes approximately 90 minutes**. This cycle is repeated throughout the night, with the length of REM periods increasing, and the length of delta sleep decreasing, until during the last few cycles there is no delta sleep at all.

So you sleep for less than 90 min you don't complete a full cycle and without a few cycles you don't feel rested because your brain cleanup is not done. 

>And why do some people require more sleep than others?

Genetics. Nothing you can do about it.