Deep sleep is the deepest stage of sleep from which it is difficult to awaken a person. When you sleep, certain changes take place in your body and brain in a cyclic fashion. These cyclic changes constitute the sleep cycle.

Providence has designed these changes to happen in such a rotational manner so as to revitalize the body and brain when you sleep. Deep sleep is one important part of the sleep cycle. Getting enough of it is important for the growth of the body.

Man has tried to understand these cyclic physiologic changes but has not yet succeeded fully.

However, there are two main stages of the sleep cycle:

  1. The Non-Rapid Eye Movement stage (NREM), and
  2. The Rapid Eye Movement stage (REM)

Ideally, an adult should get 7 to 9 hours of sleep every night. In this period, adults typically complete at least four or five sleep cycles per night.  A complete sleep cycle lasts an average of 90 to 110 minutes.

The NREM stage comprises about 80% of your sleep hours and takes up 4 to 6 hours. It forms an important and major part of the sleep cycle. The REM stage lasts for about 2 hours.

Being a longer stage, the NREM sleep is divided again into three sub-stages:

  • 1st stage or N1,
  • 2nd stage or N2, and
  • the 3rd stage or N3.
  • The 4th stage is the stage of REM sleep.

What is Deep Sleep?

Deep sleep, as the name suggests, comprises of deep slumber from which it is difficult to be awakened and if you are, you feel groggy and disoriented. It occurs during the 3rd stage of NREM sleep also called N3.

The threshold of arousal is highest during this time. It is almost impossible to wake up children during this stage. This is explained by the fact that the blood is directed away from the brain to the muscles during this stage.

It is also referred to as the stage of delta sleep because of the exclusive high amplitude low-frequency delta waves that are seen on EEG during this stage. Deep slow brain waves called delta waves emerge during this stage combined with fast waves.

The stage of deep sleep lasts for about 30 minutes during each cycle. Sleepwalking and bedwetting occur toward the end of this stage. This stage is important for a satisfying sleep, which makes you feel refreshed in the morning.

Deep sleep is most in the first two sleep cycles of the night with the maximum being during the first cycle. As the night progresses, this stage time is replaced by stage 2 of the NREM stage, and as morning approaches there is an increasing amount of REM sleep is seen.

Having a proper deep slumber greatly reduces the sleep drive, which may occur during the day if you have not slept well. Sleep drive is the amount of a person’s biological need for sleep.

A night of good deep sleep makes you feel refreshed. If you do not feel invigorated in the morning, you can be sure you have missed the required amount of deep sleep.

Tissue repair proceeds at a great pace during this stage. This is the stage of physical growth in children aided by the release of the Human growth hormone in the first deep sleep phase of the night.

Importance and function of deep sleep

Though each stage is important, stages 2 and the stage of deep sleep are particularly seen as more important. Insufficient hours can produce the most damaging effects of sleep deprivation. Important physiological processes occur during this stage.

The growth hormone is released during this stage of deep sleep, which promotes tissue and cell repair. In this stage, the repair of body tissues is at its maximum, and energy built up for the next day takes place in this stage.

This stage helps in the maintenance of good health, repair of body muscles and tissues, stimulation of growth and development of the body, and building up a good immune system.

How much deep sleep do you need?

The NREM stage comprises about 80% of your sleep hours and takes up 4 to 6 hours. The REM stage lasts for about 2 hours and utilizes about 20% of your total sleep.

Of the total duration, 15 to 25 percent is deep sleep in an adult. However, it decreases with advancing age.

If you are young under the age of 30 years, you may have 2 hours of deep sleep every night. If are above the age of 65 years, your deep sleep may last for half an hour only, or you may miss out on that totally.

Young people in their growing years require more of this sleep because it is necessary for the growth and development of the body.

Older people missing out on this sleep does not necessarily indicate a disorder.

Factors that disturb deep sleep

  • Being awakened at night by external factors such as noise.
  • Working night shifts can disturb sleep patterns because sleeping during the day can disturb you due to light, noises, and other factors.
  • Jet lag
  • Taking alcohol and smoking before going to bed will disturb your sleeping pattern. Avoid these substances before going to bed.
  • Pain and anxiety due to diseases such as osteoarthritis, GERD, premenstrual syndrome, depression, etc.
  • Aging
  • Stress
  • Certain medications and substances such as caffeine, antihistamines, antidepressants, beta-blockers, alpha-blockers, etc can disturb your deep sleep. It is important to note here that though benzodiazepines such as Valium, Ativan, and Klonopin make you easily go to sleep and lengthen your sleep duration, they are known to reduce the duration of the deep sleep stage. These drugs are used to induce sleep, to relax, for loss of memory, to reduce anxiety, to treat panic disorders, seizures, and alcohol withdrawal.

Effect of afternoon sleep on  the deep stage

Studies have also shown the best time to take a nap is in the early or mid-afternoon when you naturally experience a decline in energy and alertness.

An afternoon nap of about 20 minutes or so does not affect your falling to sleep at night as the short siesta is mostly of stage 2 and not deep sleep.

A long afternoon siesta brings about N3 or deep sleep, which rapidly decreases sleep drive and makes it difficult to fall asleep at night.

If you take a nap in the morning, your sleep is primarily light NREM sleep. However, a nap late in the evening will comprise more deep sleep. This can disrupt your ability to fall asleep at night. You should therefore avoid taking a nap late in the day. 

How to increase deep sleep?

Some factors can increase your deep sleep hours

  • Body heating as by being immersed in a hot bathtub
  • Ingestion of a high carbohydrate meal just before bedtime
  • Intensive and prolonged exercise
  • Ingestion of Tetrahydrocannabinol (a psychotic substance found in the cannabis plant), SSRIs (antidepressants), and such other drugs increase deep sleep duration. An increase in this duration due to these drugs leads to a decrease in REM stage duration. Repeated decreases in REM sleep over subsequent nights can lead to REM rebound when the causative factor (such as the above-mentioned drugs) is removed. This can lead to symptoms of depression and bipolar disorder.

Sleep inertia

Sleep inertia is a temporary disorientation and impaired cognitive function that occurs in a person for about 30 minutes when you wake him up from deep sleep. The person experiences sleep drunkenness and it is unsafe for such a person to drive. 

The person can show slower reaction time, loss of short-term memory, and slower speed of thinking. You will not see this in other stages. This can typically last from 30 to 60 minutes, but researchers have observed it to last for 2 hours.

Longer periods of sleep inertia are especially seen in night-shift workers after they took an hour-long nap during the early morning hours.

Taking 100 mg. of caffeine after your nap reduced the duration of sleep inertia, making you more reactive more quickly. Washing your face will also help restore alertness more quickly after the nap