Research indicates that optimal sleep is between seven and nine hours. The caveat on it is the quality of your sleep. If you are getting nine hours of poor quality/interrupted sleep compared to getting six and a half hours of quality/consolidated (all together) sleep, you will potentially perform better with less sleep. Some people do better with a shorter or compressed sleep window because it allows them to have better quality sleep. Some individuals sleep deep and are able to maintain that for longer than nine hours.
There is a magic sweet spot that balances quality and quantity that you'll want to experiment with to find for yourself. Most people try to get away with as little sleep as possible so that they can do other tasks in their day, because pretty much everything trumps sleep. If you start on the lower end and think, "Wow, I can function great on only four hours of sleep," you are likely mistaken. The problem with that type of self-assessment is that we are horrible at being able to assess our own level of functioning. It's kind of like walking around with the blood alcohol level of a few drinks in your system at all times. When that becomes your "norm," you just get used to it and think you are functioning fine. Kind of like a functional alcoholic. Although we would never call you a functioning-sleep-deficit-aholic, but that's essentially what you are.
Here's what you need to do to decide if six hours of sleep is enough for you. This is a self-experiment that anyone can do. If you want more support with it be sure to discuss this with your doctor or a sleep specialist. Keep a detailed sleep log that tracks time asleep, time awake, sleep quality, time in bed, and sleep efficiency. Track your sleep every night, not just when you feel like it, and calculate your sleep efficiency. The goal is to maintain eighty-five percent efficiency (quality). Once you are able to consistently sleep at eighty-five percent every night, increase your sleep time by fifteen minutes. Again, track your sleep and see if you can maintain eighty-five percent efficiency. You'll hit a spot where you'll notice that once you go over a certain time (let's say seven and a half hours), where you start to wake up more during the night or wake up feeling more tired. Then back up your sleep time by about fifteen minutes and that will be your most optimal zone for sleep (in this example seven hours and fifteen minutes). Your need for sleep will also vary depending on other factors like your stress level, activity level, general health (are you sick?) etc., so be flexible enough to know that sometimes you just might need a bit more sleep than usual.
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