When I was in my twenties, I didn't think much about sleep. I felt that if I ate well, trained hard, and took the right supplements, I was good to go with health and fitness.

Today, I realize that sleep is often the deciding factor in whether or not someone succeeds.

When you think about it, most people should spend a third of their life sleeping. When you get that third right, the other two-thirds are more fun, productive, and healthier than they'd otherwise be.

Here's everything you need to know to master the art of sleep.

Article ShortcutsThe Phases of SleepHow Much Sleep Do You Need?What Happens During Sleep?Consequences of Sleep DebtCauses of Sleep Debt, and What You Can Do About ItFrequently Asked QuestionsSleep Like a Boss

The Phases of Sleep

You aren't conscious of it, but while you sleep, your body works hard to physically and mentally rebuild and repair itself.

Like the foreman on a job site, who directs and organizes electricians, carpenters, and plumbers in a methodical process, so they don't interfere with one another's work, your body divides sleep into different segments, so it can effectively address one part of the body, and then move onto another.

Throughout the night, your brain cycles through three stages of sleep: light, deep, and rapid eye movement (REM) sleep.

Each cycle lasts about 90 minutes. With normal sleep, you go through four to six cycles per night. The time you spend in deep and REM sleep, not just the amount of time you spend in bed, determines the quality of your sleep.

Most adults sleep in one, large block of time, which is also called a "monophasic" sleep pattern. Children and the elderly do best with a “biphasic” sleep pattern, meaning they need a nap in the middle of the day to complement their nighttime slumber.

Light Sleep

During light sleep, you’re semi-conscious.

You’re aware of your environment but only respond to something unexpected, such as the creaking of a door or your spouse whispering seductively in your ear.

You have enough awareness to make sense of what’s happening but are asleep enough that you don't move unless you're alarmed.

When you're under a heavy amount of stress, you can spend most of your night in this phase, and miss out on the benefits of the next two phases.

Deep Sleep

Deep sleep supports the growth and repair of your body.

Provided you remain in light sleep without interruption, 10-30 minutes later, you enter deep sleep.

Deep sleep supports your body’s physical recovery, whereas REM sleep supports your brain and mind.

You even lose your ability to regulate your body temperature, which is why it’s so important to sleep in a cool bedroom whenever possible.

During deep sleep, growth hormone rises, supporting tissue repair, fat metabolism, and many other positive health effects.

Though dreams sometimes occur during deep sleep, it is rare.

In a typical night, you enter deep sleep three to five times.

Rapid-Eye Movement (REM) Sleep

REM sleep supports the growth and repair of your brain.

The last third of the night, assuming you get a full night’s sleep, is dominated by REM sleep. In total, it makes up 20-25% of your sleep time.

REM, which follows deep sleep, is your dream state (it was also a great band in the '80s and '90s).