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      An hourglass mechanism controls torpor bout length in hibernating garden dormice


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          Hibernating mammals drastically lower their rate of oxygen consumption and body temperature ( T b) for several weeks, but regularly rewarm and stay euthermic for brief periods (<30 h). It has been hypothesized that these periodic arousals are driven by the development of a metabolic imbalance during torpor; that is, the accumulation or the depletion of metabolites or the accrual of cellular damage that can be eliminated only in the euthermic state. We obtained oxygen consumption (as a proxy of metabolic rate) and T b at 7 min intervals over entire torpor–arousal cycles in the garden dormouse ( Eliomys quercinus). Torpor bout duration was highly dependent on mean oxygen consumption during the torpor bout. Oxygen consumption during torpor, in turn, was elevated by T b, which fluctuated only slightly in dormice kept at ∼3–8°C. This corresponds to a well-known effect of higher T b on shortening torpor bout lengths in hibernators. Arousal duration was independent from prior torpor length, but arousal mean oxygen consumption increased with prior torpor T b. These results, particularly the effect of torpor oxygen consumption on torpor bout length, point to an hourglass mechanism of torpor control, i.e. the correction of a metabolic imbalance during arousal. This conclusion is in line with previous comparative studies providing evidence for significant interspecific inverse relationships between the duration of torpor bouts and metabolism in torpor. Thus, a simple hourglass mechanism is sufficient to explain torpor/arousal cycles, without the need to involve non-temperature-compensated circadian rhythms.


          Summary: In hibernating garden dormice, torpor bout length depends on oxygen consumption. This indicates that torpor duration is determined by accumulation of a metabolic imbalance, which is cleared during periodic rewarming.

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              Timing of human sleep: recovery process gated by a circadian pacemaker.

              A model for the timing of human sleep is presented. It is based on a sleep-regulating variable (S)--possibly, but not necessarily, associated with a neurochemical substance--which increases during wakefulness and decreases during sleep. Sleep onset is triggered when S approaches an upper threshold (H); awakening occurs when S reaches a lower threshold (L). The thresholds show a circadian rhythm controlled by a single circadian pacemaker. Time constants of the S process were derived from rates of change of electroencephalographic (EEG) power density during regular sleep and during recovery from sleep deprivation. The waveform of the circadian threshold fluctuations was derived from spontaneous wake-up times after partial sleep deprivation. The model allows computer simulations of the main phenomena of human sleep timing, such as 1) internal desynchronization in the absence of time cues, 2) sleep fragmentation during continuous bed rest, and 3) circadian phase dependence of sleep duration during isolation from time cues, recovery from sleep deprivation, and shift work. The model shows that the experimental data are consistent with the concept of a single circadian pacemaker in humans. It has implications for the understanding of sleep as a restorative process and its timing with respect to day and night.

                Author and article information

                J Exp Biol
                J Exp Biol
                The Journal of Experimental Biology
                The Company of Biologists Ltd
                1 December 2021
                9 December 2021
                9 December 2021
                : 224
                : 23
                : jeb243456
                Research Institute of Wildlife Ecology , Department of Interdisciplinary Life Sciences, University of Veterinary Medicine , Savoyenstrasse 1, A-1160 Vienna, Austria
                Author notes

                These authors contributed equally to this work

                [ ]Author for correspondence ( thomas.ruf@ 123456vetmeduni.ac.at )
                Author information
                © 2021. Published by The Company of Biologists Ltd

                This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution and reproduction in any medium provided that the original work is properly attributed.

                : 4 September 2021
                : 8 November 2021
                Funded by: Austrian Science Fund;
                Award ID: 27264
                Award ID: 31577
                Research Article

                Molecular biology
                cycles,interbout euthermia,metabolic rate,periodic arousal
                Molecular biology
                cycles, interbout euthermia, metabolic rate, periodic arousal


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