Sleep: Roles of Thermoregulation, Energy Conservation & Restitution
Summary
Questions
Energy conservation and sleep
Sleep & homeothermy
Sleep deprivation and temperature.
Mechanisms of thermoregulation
Circadian Rhythms, Temperature & Sleep
Heat's influence on sleep
Hibernation, torpor and cold's influence on sleep
Sleep & Restitution
Conclusions and remaining questions
Questions
Does sleep serve for energy conservation?
Does sleep serve a thermoregulatory function?
What does sleep do for thermoregulation?
Do high temperatures induce sleep?
Do low temperatures induce sleep?
Why do hibernating animals show increased SWS during bouts and after arousal?
Sleep for energy conservation?
- Smallest animals have the highest metabolic rate (i.e. energy consumption per kg body weight): small shrew eats its body weight each day, elephant eats 1/20th of its own.
Fig. 7.3 Horne 1988
Taylor et al, Duke (1970): Oxygen consumption increases linearly with running speed within each species analyzed, from mouse to dog. Fig. 7.4 Horne 1988
Pearson, Harvard (1947): Oxygen consumption of mice decreases by over 50% by huddling. Fig. 7.5 Horne 1988
Small mammals cannot show useful amounts of relaxed wakefulness. They spend most of their awake life moving. Perhaps sleep is the easiest way to immobilize them; in any case it seems to be the only way they have.
No measurement has been done to establish energy saving during sleep with respect to wakefulness in a small mammal.
In cows and sheep, 10% and 13%, respectively, decrease in oxygen consumption from relaxed wakefulness to sleep. 9-12% in humans at thermoneutrality excluding the change due to circadian rhythm (Fraser et al., 1989).
Exercise per se does not increase or change subsequent sleep (Horne et al; see below).
But…
- The metabolic rate (MR) of naked white human exposed to Tamb of 21 deg C decreased by as much as 40% after sleep onset (Palca, Walker & Berger, 1986). Australian aborigines slept naked at temp. near freezing, and displayed MR decline (Hammel, … & Milan, 1959).
- Small birds downregulate temperature during sleep more when fasted (Heller et al, 1989; Reinertsen et al, 1986; Berger et al, 1991).
Fig. 3 Berger 1995
Deviations in habitual sleep time produce performance losses equivalent to those produced by shortened sleep (Taub & Berger, 1976), suggesting the possibility that sleep deprivation effects may be more due to the disruption of circadian rhythms associated with sleep than to the sleep loss itself.
Conclusions
: Small animals may well benefit from a substantial energy saving from sleep. Whether this is significant for humans is controversial.
Sleep as we know it occurs in mammals and birds
Mammals and birds are homeothermic animals.
If the fundamental functions of sleep evolved with homeothermy, we should find that reptiles can live without sleep.
Sleep deprivation on reptiles? SWS might increase slightly after sleep deprivation by continuous movement in aquatic turtle (Lazarev, 1981).
More on reptile sleep in Evolution of Sleep lecture.
Sleep deprivation induces temperature fall
1 week of sleep deprivation in rats causes an increase in food consumption and a drop in body temperature even as the rats attempt to keep themselves warmer.
In humans, one of the first thing to happen, even after a single night of sleep deprivation, is a subjective feeling of cold.
Sleep deprivation in humans causes a small fall in body temperature starting on the first night, although the circadian rhythm remains (Patrick & Gilbert; Horne). Effect may be larger if clothing is kept constant. Figs. 3.1 & 3.2 Horne 1988
Sleep deprivation induces decrease in heat loss by sweating (Sawka et al).
Mechanisms of Thermoregulation
Temperature is regulated at a set point via homeostasis.
Hibernation, sleep and fever are changes of the set point, with shared mechanisms. Because of its clinical importance, fever is perhaps the best understood.
During hyper/hypothermia, in contrast, the body works actively to restore the lost set point.
A lesion of the cervical cord disrupts fever response, but heat production and heat exchange remain relatively unaffected (Claude Bernard, 1876).
Preoptic area of anterior hypothalamus (POAH) is crucial for fever induction.
Cytokines (e.g. interleukin 1 [IL-1] and IL-6) are endogenous pyrogens (fever inducers), changing set point.
Thermosensitive cells are located deep in the body, skin, spinal cord and hypothalamus.
4-13% of anterior hypothalamic neurons are cold-sensitive and 10-32% are warm-sensitive. They modulate their firing rates at 0.6-0.8 Hz/deg C.
Molecular mechanisms of thermosensitivity are poorly understood. Arrhenius' law states that rate of a chemical reaction doubles upon warming by 10 deg C. Neuronal are much more sensitive than this, suggesting the usage of amplification cascades and/or autocatalytic loops.
POAH thermosensors project to nucl. Stria terminalis, and from there to brain stem, and thus through sympathetic system create increased heat production, oxygen uptake, heat conservation.
Mechanisms of heat production and conservation include metabolic rate, shivering, sweating, panting, vasoconstriction/dilation, changes in the body surface.
To manage higher thermogenesis required in small mammals, newborn humans and rodents have specialized tissue for heat production: brown fat depots. Cells there express the uncoupling protein, which uncouples ATP production from respiration in mitochondria, leading to heat production.
Set point can be altered with drugs such as aspirin, which reduces fever by 2-3 deg C but does not, however, alter the set point during non-fever normal temperature.
Does aspirin have any effect on the set point change during sleep?
Set point shows a circadian rhythm: .7 to .8 deg C lower during night in humans.
Not all animals sleep at night. Nocturnal rodents display an inverted periodicity, with lower temperatures during the light period. This makes it clear that the temperature rhythm is associated with sleep and wakefulness or activity and inactivity, but not with external temperature or light.
Circadian Rhythms, Temperature & Sleep
'Time-free' experiment (Czeisler et al.; Zulley et al):
- Body temperature rhythm soon started cycling with 24.5 hs period; sleep rhythm continued on a 24-hr cycle for ~2 weeks.
- After 10-14 days, half the subjects' sleep cycle became uncoupled from the temperature rhythm, while half maintained them synchronized.
- Sleep onset at time of temperature minimum leads to short sleep (8 hs); sleep start at temperature peak leads to long sleep (14 hours).
Fig. 6.4 Horne 1988
86% of awakenings occurred on the rising phase of the temperature rhythm (in normal conditions, morning).
- Keeping warm does not ameliorate sleep deprivation symptoms in rats
(Shaw & Rechtschaffen, 1997):
- Rats who can regulate ambient temperature upregulate it to 9 deg C above baseline during total sleep deprivation (TSD).
- True even while brain temp. was above baseline.
- Demonstrates TSD induces an elevation in temp. set point.
- Nonetheless, rats lose temperature and die.
- Surprisingly, survival time was shorter than in previous studies.
Conclusion: Although temperature and sleep seem closely linked, this may be just due to a necessity of higher temperatures for activity and that of having sleep occur during inactivity. It appears improbable that the noxious effects of sleep deprivation are due to increased energy expenditure or even to temperature changes.
Neural Basis of the Thermoregulatory Effects of Sleep Deprivation
Bilateral POAH lesions sufficient to impair homeothermic responses to changes in Tamb did not produce TSD-like temperature changes, suggesting thermoregulatory changes produced by TSD do not result from POAH impairment (Feng, Bergmann and Rechtschaffen, 1995).
Lesioned rats show earlier, steeper, and greater declines in brain temperature, suggesting that in unlesioned rats the POAH may counter-regulate against the reduction of heat retention caused by TSD.
Lesioned rats did not show the characteristic TSD-induced early increases in brain temperature, suggesting that the Tset increase in unlesioned rats is POAH-mediated or that heat retention is so compromised in lesioned rats that temp. could not rise despite an increase in Tset.
Diurnal Heat induces increases in SWS in humans & rats but not in squirrels
A warm bath induces 25-33% increase in SWS (Horne et al, 1983, 1985).
Increased metabolic rate may cause an increase in adenosine production, which in turn causes sleep (Brundegge et al, 1996; Porkka-Heiskanen et al, 1997).
Heat induces SWS in rats (Obal et al, 1995).
Squirrel TST, REM & SWS were unaffected by Tamb varying between 22 and 35 deg C (Walker, … & Berger, 1983).
Thermoregulation ceases during REM sleep
- Brain temperature increased in Tamb above thermoneutrality and decreased for Tamb below thermoneutrality (Walker, … & Berger, 1983).
Torpor and hibernation
- Food restriction usually causes shallow torpor or hibernation, even in summer for desert-living animals (aestivation).
- Torpor and hibernation are usually entered through sleep, suggesting possible common origin. Occurs fairly quickly after sleep onset (brain temp. falls to 10 C within hours and REM sleep disappears).
- Ground squirrels show increased SWS after long hibernation bouts as compared with short ones (Trachsel, Edgar & Heller, 1991,
Figs. 3 & 4; Canguilhem & Boissin, 1996, Figs. 4 & 5), saturating after ~ 6 days.
If hibernating animals warm up during arousals in order to sleep, sleep cannot be considered solely a strategy for saving energy.
The same increase in SWS is true for sleep EEG after daily torpor in hamster (Deboer & Tobler, 1994).
The increase in SWS is only seen after hibernation in cold (-5 to 0 deg C) and not warmer (10 deg C) ambient temperatures (Larkin & Heller, 1996; Strijkstra & Daan, 1997). There is no relationship between SWS & bout duration in euthermy.
Electrophysiology of hibernating animals at 14-36 deg C shows alternation of NREM & wake-like states (Heller et al, 1988).
Conclusion: SWS's role cannot occur in the cold.
Sleep & Restitution: Mental & Bodily Exercise
Duration of SWS increases linearly with prior wakefulness. Figs. 5.1 & 5.2 Horne 1988
Slope increases after one waking day (16 hours).
Time of day has little influence on SWS.
REM duration in 1st 4 hours of sleep decreases linearly with prior wakefulness.
Minimum sleep for normal performance in a detection task after 1 night of reduced sleep is 3 hs. (Wilkinson). There is little REM during first 3 hours of sleep. Fig. 2.5 Horne 1988
Minimum sleep for normal performance in a detection task after several nights of reduced sleep is 5 hs. (Wilkinson).
Conclusion: SWS and not REM is likely to mediate restitution of vigilance after prolonged wakefulness.
Marathon runners in S. Africa experience SWS increases after race; marathon runners in Sweden do not.
Exercise during sleep deprivation does not affect sleepiness or performance (Webb & Agnew; Lubin et al; Angus et al).
Exercise in the cold does not produce any sleep changes (Horne et al., 1983, 1985).
- Stimulation (busy day of sightseeing) causes sleep-deprived subjects to feel more sleepy, fall asleep faster, sleep longer (11 hs vs 10 in control) and show 40% more SWS rebound (Horne).
Conclusion: SWS's restorative effect is likely to relate to cerebral wear and tear more than to bodily one.
References
- Berger & Phillips (1995): Energy conservation and sleep.
- Canguilhem & Boissin (1996): Are the animals in deep hibernation awake?
- Daan et al (1991): Warming up for sleep? - ground squirrels sleep during arousal from hibernation.
- Deboer & Tobler (1994): Sleep EEG after daily torpor in the Djungarian hamster: similarity to the effects of sleep deprivation.
- Feng, Bergmann and Rechtschaffen (1995): Sleep deprivation in rats with preoptic-anterior hypothalamic lesions.
- Horne & Staff (1983): Exercise and sleep: Body-heating effects
- Horne & Moore (1985): Sleep EEG effects of exercise with and without additional body cooling.
- Horne & Reid (1985): Night-time sleep EEG changes following body heating in a warm bath.
- Horne (1988): Why we sleep.
- Kandel, Schwartz & Jessel (1991): Principles of Neural Science, 3rd edition.
- KRILOWICZ BL; GLOTZBACH SF; HELLER HC (1988): NEURONAL-ACTIVITY DURING SLEEP AND COMPLETE BOUTS OF HIBERNATION
- Larkin & Heller (1996): Temperature sensitivity of sleep homeostasis during hibernation in the golden-mantled ground squirrel.
- LAZAREV SG (1981): SLEEP-DEPRIVATION IN THE REPTILE TESTUDO-HORSFIELDI - THE COMPARATIVE-PHYSIOLOGICAL ASPECT. DOKLADY AKADEMII NAUK SSSR 261: (6) 1492-1495.
- Miller & South (1981): Entry into hibernation in M. Flaviventris: Sleep & Behavioral Thermoregulation.
- Obal et al (1995): Promotion of sleep by heat in young rats.
- Porkka-Heiskanen et al. (1997): Adenosine: A Mediator of the Sleep-Inducing Effects of Prolonged Wakefulness.
- Shaw, Bergmann and Rechtschaffen (1997): Operant control of ambient-temperature during sleep-deprivation.
- Strijkstra & Daan (1996): Sleep during arousal episodes as a function of prior torpor duration in hibernating European ground squirrels.
- Strijkstra & Daan (1997): Ambient temperature during torpor affects NREM sleep EEG during arousal episodes in hibernating European ground squirrels.
- Sundgren-Andersson et al (1998): Neurobiological Mechanisms of Fever.
- TRACHSEL L; EDGAR DM; HELLER HC (1991): ARE GROUND-SQUIRRELS SLEEP-DEPRIVED DURING HIBERNATION ?
- Walker, … & Berger (1983): Cessation of thermoregulation during REM sleep in the pocket mouse.
By Alex Bäcker, Feb. 1999.