Length: 5 to 10 minutes
N1 is the first and lightest phase of sleep, when you feel yourself just drifting off to slumber land. Unfortunately, this stage can be annoying for your bed buddy because it features sudden twitches called hypnic jerks, often along with a sensation that you're falling. You may also experience eerie hypnagogic hallucinations during this stage, such as seeing moving shapes or hearing someone calling your name--so thankfully it's a short phase!
Length: 10 to 25 minutes
N2 is when you're considered to be asleep for the first time. Your brain activity, heart rate and breathing start to slow down, and your body temperature falls a little. You spend most of your night in this light phase of sleep.
Length: 20 to 40 minutes
N3 is your deepest sleep of the night. During this phase your brain activity, heart rate and breathing are at an all-time low. This is a very important stage when your body repairs, regenerates and strengthens your immune system. Don't be surprised if funny sleep activities like sleepwalking and sleep talking happen in this phase--and if you have a child, bedwetting. It's no easy task to wake someone up during N3, but if you do you could have a tired, grumpy and groggy person on your hands for several minutes.
Length: 10 Minutes
Before you head into Stage R, you usually pop back up to N2 for 5 to 10 minutes. As the night progresses each REM sleep lengthens, with the final one lasting up to an hour. This is that fascinating time of the night when you do your most intense dreaming. Your blood pressure, breathing and brain activity increase, and your body temperature plummets. Interestingly, the muscles in your arms and legs also become paralyzed; scientists think this prevents us from acting out our dreams.
After REM sleep, you usually head back to N1 or N2 and start the whole cycle over again. With each passing cycle you spend more time in light sleep than deep sleep, which gets you ready to rise and shine in the morning!