I was going to say that also. Slow-wave (deep) sleep. That’s when the natural cleaning process occurs.
Some animals, such as dolphins and birds, have the ability to sleep with only one hemisphere of the brain, leaving the other hemisphere awake to carry out normal functions and to remain alert.
I knew there was something different about the daytime "nap" sleep my small parrots experience. It gives the impression that they are asleep yet alert at the same time, can react instantly, but getting the benefit of a nap just the same. This is why.