Interesting though that those with mood disorders (depression, specifically) actually spend more time in REM sleep than do those without mental illness.
Seriously? How did they test such a thing lol
Even more interesting is that studies were done where they woke people up during different stages of sleep to record their dreams, and REM sleep was significantly correlated with more nightmares than earlier stages.
Somehow that's not surprising. The deeper in your subconscious mind you go, the less in control you are.
They tested through EEG. Different stages of sleep are marked by different electrical wave signatures (e.g. delta waves). It's as simple as timing how long a sleeping individual's brain emits each kind of wave signature.