Evolutionary Function of Sleep

Different Species

• For herbivores, the higher their weight, the less they sleep

Duration of Sleep in Animals

• There is no obvious correlation between sleep duration and ability/intelligence

• Longest sleepers = koalas

• Shortest sleepers = elephants

Unihemispheric Sleep

• Dolphins sleep 1 hemisphere at a time because they need to remember to breathe

• Sea otters sleep 1 hemisphere at a time in the sea but 2 hemispheres at a time on the land

• Some birds sleep 1 hemisphere at a time in long-distance flights

Theories of Sleep Function

1. Host-Defense

• Sleep loss = lower immune defense

  • Fewer immune system cells (white blood cells)

  • Some immune markers are affected by even 1 day of sleep deprivation

• Sleep changes with disease:

  • We sleep more

    • Maybe to conserve the precious energy required to fight the disease

  • Increase in NREM sleep + decrease in REM sleep

  • Decrease in NREM sleep bout length → sleep fragmentation because we wake up more often

  • More heat production → fever

2. Conservation of Energy

• Sleep has a metabolic benefit for the whole body

• There is less glucose use in NREM sleep

• Sleep-wake cycle: schedule of energy investments

  Wake	Sleep
  Waking Effort (WE): Vigilance, Reproduction, Foraging Effort	Biological Investment (BI): Growth, Maintenance, Repair, Immune Function, Neuronal Network Reorganization
  Thermoregulatory Effort (TE): Increased body temperature

Caloric Conservation

• Sleep is involved in the conservation of calories:

  • Lower body temperature in sleep

  • Hibernation

  • Reduced energy use

• Short sleep is associated with:

  • Risk of diabetes

  • Obesity

  • Cravings for carbs

  • Lower glucose tolerance

3. Glymphatic Function: Removing Toxic By-Products

• The glymphatic system, which serves a waste clearance function for the CNS, is more active in sleep

  • Glial cells eliminate neurotoxins such as amyloid beta

• Astrocytes create fluid-conducting channels that facilitate the distribution of important molecules for brain functioning

  • Influx = lipids and other molecules

  • Outflow = metabolic waste

4. Connectivity/Plasticity Function

• Wake = new synapses

• Sleep = remove unnecessary synapses

Synaptic Homeostasis Hypothesis - Tononi

1. New learning happens primarily by synaptic potentiation

2. Synaptic potentiation occurs primarily in wake when the organism interacts with the environment, not in sleep when it is disconnected

3. Renormalization of synaptic strength happens primarily during sleep when the brain is spontaneously active off-line, not in wake when a neuron’s inputs are biased by a particular situation

   • This renormalization and downregulation of synapses happens during NREM sleep and is shown through cortical slow waves and hippocampal sharp waves

• Wake: sensory input → current sampling + synaptic potentiation

• Sleep: sensory disconnection = comprehensive sampling + synaptic down-selection

5. Restoration of performance

• Sleep helps to restore cognitive functions such as:

  • Memory

  • Reflexes

  • Attention

  • Mood

  • Emotional reactivity

  • Emotion

New Theories

REM Sleep and Vision

• REM sleep internally generate activity that prevents occipital brain areas from being inactive

Developmental Shifts

• Sleep is for neural reorganization until 2-3 years of age

• Sleep is then for repair and clearance

General Information

• Sleep most likely serves many functions

• Sleep appears in any organism with a neuronal-glial network

• Sleep might be an emergent property because it comes out of the system but it not explained by the system itself

• We don’t know if plants sleep because sleep is defined as a brain state and they don’t have a brain

• Most animals have some type of quiet sleep

• New neurons grow in the hippocampus even in adulthood

• Chronic sleep disruption may lead to less neurogenesis because it makes it harder to make new memories and encode them