Why do we sleep?
It might surprise you to learn that there’s still no hard and fast answer to this question.
Why not? Sleep medicine as a research and clinical focus is still rather young, with key discoveries about sleep occurring mostly in the middle to late 20th century. If sleep researchers don’t know the answer, it’s because there simply hasn’t been enough research to confirm all the theories out there.
What the public may not always understand is that research requires funding, and funding doesn’t happen without a sense of urgent “need to know” as well as the enthusiasm from both the people and the government.
Only until recently has it become clear to everyone in the medical field that sleep is not a passive, secondary process, but rather a dynamic, necessary one for overall health and well being.
Theories about why we sleep
This said, there are some theories which could sensibly explain the purpose of sleep:
We sleep to allow the body to recover from a day’s worth of stress
Sleep provides the body a chance to manage physical and emotional stress accumulated throughout the day. Scientists continue to see distinct links between sleep and recovery for athletes, people experiencing large amounts of emotional stress, even the functionality of the immune system in relation to quantity of sleep.
- JUL 2018 || Journal of Adolescence: The role of sleep in adolescents’ daily stress recovery: Negative affect spillover and positive affect bounce-back effects
- JUL 2018 || MedicalXPress: Antioxidant benefits of sleep
- JUN 2018 || American Psychological Association: Stress and Sleep
- APR 2018 || International Journal of Sports Physiology and Performance: Sleep-related Issues for Recovery and Performance in Athletes
- JUL 2008 || Sleep Medicine Reviews: Restricted and disrupted sleep: Effects on autonomic function, neuroendocrine stress systems and stress responsivity
- JUL 2006 || Nursing and Healthcare Management and Policy: Disentangling the effects of psychological and physical work demands on sleep, recovery and maladaptive chronic stress outcomes within a large sample of Australian nurses
We sleep to allow the brain to consolidate all the information it has processed during the day
Sleep seems to offer the brain a chance to process everything experienced and learned throughout the day. Consolidating memory in particular (much like “defragmenting” one’s computer) helps keep the brain running efficiently; getting adequate sleep, for this reason, can be linked to better memory health as well and learning performance overall.
- SEP 2018 || VeryWell: How Consolidation Turns Short Term Memories Into Long-Term Ones
- JUL 2018 || PLoS–Computational Biology: Differential roles of sleep spindles and sleep slow oscillations in memory consolidation
- MAR 2018 || MedicalXpress: Cueing newly learned information in sleep improves memory, and here’s how
- SEP 2016 || Sleep Medicine Review: Memory Consolidation in Sleep Disorders
- OCT 2005 || Nature: Sleep-dependent memory consolidation
- OCT 2003 || Journal of Sleep Research: Memory consolidation during sleep: a neurophysiological perspective
We sleep to “recharge the battery”
By being awake for around 16 hours each day, we generate a “sleep drive” not unlike an appetite after having not eaten. This sleep drive pushes us to turn in for the night to recharge and reenergize ourselves.
While a sleep drive might be considered an internal physiological aspect of sleep, it cooperates (or should) with the circadian system, which relies on external time cues like the earth’s light-dark cycles to create a sleep-wake pattern. This explains why you feel so tired during the day after not getting enough consolidated sleep at night.
You need to go into “sleep” mode to maintain optimal health (or, clinically speaking, homeostasis). Sleep deprivation is the often-dangerous consequence of not getting that recharge.
- NOV 2018 || National Heart, Lung and Blood Institute: Sleep Deprivation and Deficiency
- JUN 2018 || Neurobiology of Sleep and Circadian Rhythms: Sleep homeostasis and the circadian clock: Do the circadian pacemaker and the sleep homeostat influence each other’s functioning?
- MAY 2018 || Sleep Research Society: Response to chronic sleep restriction, extension, and subsequent total sleep deprivation in humans: adaptation or preserved sleep homeostasis?
- APR 2018 || SLEEP: Effects Of Sleep Deprivation And Recovery Sleep On Human Brain Network Organization
- APR 2017 || Journal of Sleep Research: The homeostatic and circadian sleep recovery responses after total sleep deprivation in mice
- DEC 1999 || Journal of Biological Rhythms: Sleep Homeostasis and Models of Sleep Regulation
We sleep to deep clean the central nervous system
When we think of waste products in the human body, we’re more apt to consider the digestive and excretory systems, or maybe the skin as an organ which clears waste (through the sweat glands). However, waste management in the body happens literally at the cellular level; each cell has its own cleanup crew to remove toxins.
The brain, made up of billions of cells, must also find a way to remove its waste products. If not, they can lead to neurological conditions like Alzheimer’s disease. Sleep provides the brain with the time needed to do the “dirty work.”
- OCT 2018 || Trends in Neurosciences: Circadian Clocks and Sleep: Impact of Rhythmic Metabolism and Waste Clearance on the Brain
- SEP 2018 || VeryWell: How Sleep Cleans Up the Brain
- JUL 2018 || Science News: The brain may clean out Alzheimer’s plaques during sleep
- DEC 2017 || USA Today: 2018 Goals: Get more sleep. Sleep deprivation is toxic to your health
- OCT 2016 || Neurotoxicity of Nanomaterials and Nanomedicine: The Distribution and Elimination of Nanomaterials in Brain
- JAN 2014 || The New York Times: Goodnight. Sleep Clean.
The brain requires sleep to remain “plastic” or flexible to process new stimuli
Neuroplasticity is a term that describes the ability of the brain to makes internal system changes, such as redirecting brain signals through alternative pathways, or cortical remapping. Plasticity can also describe the degree to which the brain can regenerate gray matter or strengthen synaptic connections.
Our brains are thought to be more or less “plastic” at all ages, though it makes sense that children and young adults, who have brains still in development, may actually enjoy more plasticity.
Sleep has been shown to provide opportunities for the brain to do the work of repairing, rebuilding, and remapping.
- JUL 2018 || Queensland Brain Institute: Martin Sale: brain plasticity and sleep
- JUN 2018 || Forbes: New Discovery Reveals How Your Brain Changes When You Need Sleep
- MAR 2018 || Neuroscience of Biobehavioral Reviews: Brain plasticity and sleep: Implication for movement disorders
- DEC 2012 || Frontiers in Neurology: NREM sleep oscillations and brain plasticity in aging
- JUN 2005 || Neuroscience: Sleep-dependent motor memory plasticity in the human brain
- SEP 2004 || Pediatric Rehabilitation: A role for sleep in brain plasticity
When will we know for certain why we sleep?
Thanks to significant gains in neuroscience and the wonderful spotlight pointed at circadian research at the Nobel prize level in 2017, we are learning much more about sleep as a physiological process at a more accelerated rate. Any headlines in the news sharing fresh discoveries about sleep are something to look forward to as researchers continue to demystify this important aspect of human health.