REM, like other stages of sleep, is a biological necessity.  When we don’t get enough REM, our brain will compensate by promoting REM whenever possible.  This is called REM Rebound.

Yes, it is possible to have a higher than average ZQ, but still have fairly poor sleep quality. There are several reasons for this.

First, ZQ is a very personal number, much like your weight. My weight tells me how I'm doing and I can use it as a gauge against what's typical for people like me, but wouldn't mean much to you. Some people need more sleep than typical, others need less. If you get a better than average ZQ, but still feel groggy, maybe typical isn't enough for you.

The formal term for banking sleep is "sleep extension."  By getting a good night's sleep, subjects of an earlier 2009 study were able to stay more alert than those who didn't bank sleep before undergoing a week straight of only 3 hours of sleep a night.  Banking sleep also led to higher scores on various cognitive tests, though don't wait until the night before an exam to start getting more.

It can take several days, if not weeks, to recover from a spell of sleeplessness. Keep in mind that habitual good sleeping is the best way to recover sleep and start feeling balanced again. Relax and give yourself time to recoup your "sleep debt," which are the hours of sleep lost that must be repaid.

A good question - there's quite a bit of science that suggests the answer is "NO", but I'll get to that in a little bit. First let's talk about different things people are trying to get by on less sleep.

First, there's the class of people who, by increasing their sleep quality, can get by with less time in bed:

We all have different sleep needs, and while some people really can live a healthy life with less sleep than others, there could be more going on here.