How much sleep do we need?
This is a common question many people ask me.
Before I answer this question let me briefly describe what sleep is.
Sleep is a highly complex and active process of
dynamic restoration, rejuvenation, recuperation and
reconsolidation, all of which are essential for health and wellbeing.
Sleep is a fundamental and highly complex biological
phenomenon. Neurotransmitters in the brain control our
sleeping patterns in interaction with neurons.
Neurotransmitters such as serotonin and norepinephrine keep
some parts of the brain active while we are awake. Other
neurons at the base of the brain begin to signal when we fall
asleep, “switching off” the signals that keep us awake.
Chemicals are central to sleep, particularly adenosine, which is
produced by our cells, accumulates in our blood and travels to
the brain while we are awake and causes what we experience as
sleepiness. This chemical gradually breaks down while we sleep
and allows us to wake. Yes, sleep has a complex biology.
The stages of sleep
Our nightly sleep is composed of a number of sleep cycles, each
of which is made up of different stages. There are two main
types of sleep – non-REM and REM sleep – and the differences
between them are almost as profound as the differences between
sleep and wakefulness. Scientists today use a variety of
technologies to understand the diverse aspects of sleep by
measuring the electrical activity of the brain, the muscles and
movement of the eye.
It is now understood that we pass through five stages of sleep as
we fall from wakefulness, with REM (rapid eye movement) sleep
the final of these stages. These stages progress in cycles, moving
through stage 1 to REM sleep and then back to stage 1. The
progress of each cycle lasts around 90 minutes in adults and 30–
40 minutes in children.
During the first stage, which lasts approximately 15 minutes, we
actually move back and forth between wakefulness and sleep.
Brain waves during this stage give way to alpha waves. In the
second stage, we enter what can be termed the first “real” sleep.
Here theta waves begin to appear as we enter a phase which
comprises 50% of our sleep time. During this stage, the body
prepares to enter deep sleep. Both stage one and two are
relatively light stages of sleep.
The third and fourth stages of sleep are termed “slow wave
sleep” and we spend approximately 20% of our total sleeping
time in these stages. Here we experience delta waves and a form
of deep sleep from which it is very hard to wake.
REM is the last stage of the sleep cycle. As rapid eye movement
begins, our skeletal muscles are virtually paralysed. The brain
wave pattern shows a blend of alpha and beta waves. During
this stage, our breathing becomes more rapid, irregular and
shallow, and our heart rate rises along with blood pressure. It is
at this stage that males develop penile erections. REM is the
stage during which we dream, often in a bizarre and illogical
The first period of REM usually has a duration of 10 minutes. Each
subsequent REM stage increases in duration, with the final one
lasting up to an hour.
Why we sleep?
It is still not understood why we evolved the need to sleep. What
is clear is that sleep, as with breathing, is a fundamental human
requirement and not somehow an optional add-on during life.
Studies into animals show that sleep is a prerequisite for
survival. Rats, for example, typically live for 2 to 3 years. Those
deprived of REM sleep survive only about 5 weeks on average
and rats deprived of all sleep stages live only 11–32 days.
Sleep-deprived rats develop a number of ailments: abnormally low
body temperature and sores on their tail and paws, to name two.
Some studies suggest that sleep deprivation affects the immune
system in detrimental ways. The sores developed by the sleep-deprived rats seem to be evidence of an immune system
imbalance caused by a lack of sleep. Sustained sleep deprivation
can lead to mental disturbance, anxiety, tiredness, fatigue and
even death if prolonged. There are reports of cases outside
scientific studies where people have died after periods of no
sleep at all. In such cases, people experience symptoms of
hallucinations, weight loss and finally dementia before their
Sleep is so important to well-being that scientists argue that
animals can survive for three times as long without food as
How much sleep do we need?
It is well known that people need approximately 8 hours of sleep
every night, but to be precise we need around 7.5 hours. As each
sleep cycle has a duration of about 90 minutes, and we need 5
such cycles, the total for recommended sleep is 7.5 hours.
Sleeping well depends not only on the number of hours, but on
the quality and consistency of sleep. Intuitively, we experience
this. One can spend 10 hours in bed and still feel tired in the
To calculate in a meaningful way the sleep we require, and to
know when we should be going to bed, we need to work
backwards and define the time at which we need to get up. For
example, if we need to rise at 7 am, we need to, firstly, count
back 7.5 hours, which brings us to 11.30 pm. Yet getting to sleep
is a process and requires sleep onset latency, a period of time
required to fall asleep after lights have been turned off, which
lays the foundation for a solid night’s sleep. This period should
be about 10–20 minutes. Allowing, therefore, for 20 minutes of
sleep latency, 11.10 pm can be defined as the time to be getting
into bed to get enough hours of quality sleep, assuming a
reasonable quality of sleep ensues.
Interestingly, if you find yourself asleep as soon as you hit the
pillow, this may be an indicator of sleep deprivation. If the onset
of sleep takes longer than 20 minutes, this may be a symptom of
Find out more about sleep and how you can get a better quality sleep in my book:
Categories Sleep for better health