SLEEP L2: Franks, Wisden, Harding. 2019. Front Neurosci - temperature/sleep

Question 1

preparation for sleep is a thermoregulatory behaviour

Answer 1

• thermoregulatory behaviours preceding sleep are conserved across mammalian species (eg. huddling, curling up, seeking warmth & shelter) –> suggesting functional role beyond comfort, likely in initiating & maintaining sleep
• mice shower preference for warmer environements during sleep, aligning their circadian temperature decline with the light-dark cycle and sleep onset
• ambient temperature significantly influences energy expenditure, & failure to engage in thermal defense behaviours can disrupt energy homeostasis
• humans regulate temperature during sleep by unconsciously adjusting their exposed surface area, aiming to establish skin microclimates between 31-35ºC, optimal for sleep; deviations from this range negatively affect sleep quality

Question 2

warm bath effect

Answer 2

• in humans, immersion in hot water 1-8hrs prior to sleep, leads to decreased sleep latency & increased sleep depth
• response to external temperature, particularly vasodilation in hands and feet, are key predictors of sleep initiation
• vasodilation, often part of the circadian temperature decline, occurs up to 2hrs before onset of the first slepe episode during wake phase
• as core temp decreases, alertness declines
• individuals are most likely to choose a moment for sleep onset when their body temp is decreasing at max rate (heart rate also decreases as sleep approaches)
• lowest core temp is typically observed 2hrs after sleep onset in humans
• in natural conditions, increased levels of circulating melatonin coincide with declining core temperature prior to sleep onset
• disruption of the circadian cycle, particularly delaying the onset of the first NREM sleep episode, can impact core temp decline –> seen in cases of sleep deprivation & delayed sleep phase disorders
• patients w/difficulty in peripheral vasodilation, eg. vasopastic disorders, tend to have longer sleep latencies
• narcoleptic patients exhibit altered proximal-to-distal skin temp gradients during waking hours, affecting sleep propensity
• raymann et al. 2008 showed that small changes in skin temp, as little as 0.4ºC within the range of 31-35ºC can shorten sleep latencies w/o affecting core temperature
  –> they used custom-made thermosuits to manipulate skin temp, and found that subtle chagnes in warmth can even promote deeper sleep in challenging groups such as elderly insomniacs –> suggests that sleep difficulties in elderly may be related to deficits in thermoregulation
• warming persists throughout the night, maintaining a sleep-permissive state while allowing selective vasodilation in NREM & vasoconstriction in REM and wake
• though body and brain cooling coincide with sleep onset, they are not shown to initiate NREM but rather result from vasodilation

Question 3

neuronal control of thermogenesis & its influence on sleep

Answer 3

-warm stimuli activate PO neurons; lesions in PO disrupt warm-defense behaviour and reduce total sleep in cats
- in rats, PO lesions alter thermal preference toward warmer temperatures conducive to sleep recovery
- warming increases delta power in EEG
- ~21% of PO neurons are thermosensitive
- warm-sensitive neurons (WSNs) often active during NREM, inhibit arousal nuclei during brain warming
- WSNs are influenced by pyrogens like prostaglanding E2

• recent experiments have shown that activating galanin neurons in the VLPO and LPO can induce both NREM sleep and hypothermia
• in LPO, deletion of galaning neurons using caspase expression abolished rebound delta power following sleep deprivation

Question 4

thermogenesis links sleep to energy homeostasis

Answer 4

• thermoregulation influences energy homeostasis, and alters feeding requirements, adding another homeostatic drive to modulate sleep networks
• following meal, adipocytes release leptin (signals excess energy intake & inhibits feeding)
• leptin receptors in PO are activated by rising ambient temperatures, leading to reduced energy expenditure and food consumption
• lectin signalling has a direct impact of sleep –> systemic administration increases both NREM and REM sleep durations, while leptin-deficient mice exhibit fragmented sleep and lower core temp

Question 5

why link NREM sleep & body cooling?

Answer 5

• one hypothesis proposed that lower temperatures coinciding with NREM sleep serve to cool the brain, reducing cerebral metabolism, conserving energy, and assisting other functions from immune regulation to circadian coordination
• while energy conservation is important, extreme hypometabolism in torpor and hibernation occurs at the expense of sleep, suggesting that energy conservation alone is not the primary function of sleep
• morphological changes in dendritic spines occur at temperatures of 20ºC or less, suggesting a more direct role of reduced temperatures in the brain