Sleep Cycle Science

Have you ever slept through the night, say, seven hours or more, and still woken up the next morning feeling groggy and out of it? Science has an explanation for this, and it revolves around sleep cycles.

The human sleep cycle lasts about 90 minutes, and during that time we routinely move through 5 stages of sleep. The first four stages are made up of non-rapid eye movement (NREM) sleep, and the final stage is when rapid eye movement (REM) sleep occurs. Our sleep cycle stages look a bit like this:

Stage1: 4-5% of cycle - Light sleep when muscle activity slows down.

Stage 2: 45-55% of cycle - Deeper sleep. Breathing and heart rate slow and body temperature drops slightly.

Stage 3: 4-6% of cycle - Deep sleep begins with the brain starting to generate slow delta waves.

Stage 4: 12-15% of cycle - Very deep sleep with rhythmic breathing, limited muscle activity and continual delta waves.

Stage 5: 20-25% of cycle - Rapid eye movement. Brainwaves speed up and most dreaming occurs. Muscles relax and heart and breathing rates increase.

As you can see from the table, we move from light sleep to very deep sleep during NREM sleep stages. The body is restored in NREM sleep as essential hormones are released to stimulate tissue growth and muscle repair.

During REM sleep we experience bursts of rapid eye movements and most of our dreaming takes place. REM sleep is when energy is replenished in the brain and the mind is functionally restored for the following day.

Since a complete sleep cycle is necessary to restore body and mind, the number of uninterrupted cycles we experience may impact our health as much, or more, than the total number of hours we spend sleeping in any given night.

When we’re able to sleep through the night, without alarm clocks, cell phones or other disturbances, we tend to wake up naturally after a multiple of 90 minute sleep cycles. Say, after 4 ½, 6, 7 ½, or 9 hours—multiples of 90 minutes—not after 7 or 8 hours which are not divisible by 90.

Have you ever woken up before your alarm feeling rested? It’s likely because you were coming out of several complete sleep cycles. Studies show that a person who sleeps only four cycles (about 6 hours) will feel more refreshed than someone who has slept 8 – 10 hours without being able to fully complete multiple sleep cycles.

Here are some practical ways to try to harness sleep cycle power for yourself.

Napping: If you feel mentally exhausted, or need to tackle a task that involves a lot of brain power, try taking a 90 minute nap to allow for a full sleep cycle. If you’re just feeling physically tired, try a 15 – 20 minute nap for a quick physical rebound.

Night Time Sleeping: If you often feel tired in the morning, try arranging your sleep schedule around 90 minute chunks. For example, if you have to be up at 6:00 a.m. the following morning try falling asleep by 10:00 or 11:30 the previous night. This should give your body the opportunity to get in 5 to 6 full sleep cycles with a buffer for waking naturally between 5:30 and 6:00 a.m.

If, after trying these tips, you find that quality sleep still eludes you, please consider calling concern at 800.344.4222, or speaking with your doctor for additional help.