Thermoregulatory model of sleep control: Losing the heat memory

Thermoregulatory mechanisms were hypothesized to provide primary control of non-rapid-eye-movement sleep (NREM). On the basis of this hypothesis, we i ncorporated the thermoregulatory feedback loops mediated by the "heat memor y," heat load, and loss processes associated with sleep-wake cycles, which were modulated by two circadian oscillators. In addition, hypnogenic warm-s ensitive neurons (HWSNs) were assumed to integrate thermoregulation and NRE M control. The heat memory described above could be mediated by some sleep- promoting substances. In this paper, considering the possible carrier of th e heat memory, its losing process is newly included in the model. The newly developed model can generate the appropriate features of human sleep-wake patterns. One of the special features of the model is to generate the bimod al distribution of the sleepiness. This bimodality becomes distinct, as the losing rate of the heat memory decreases or the amplitude of the Y oscilla tor increases. The theoretical analysis shows the losing rate of the heat m emory control's rapidity of model response to a thermal perturbation, which is confirmed by simulating the responses with various losing rates to tran sient heat loads ("heat load pulse"). The sleepiness exhibits large respons es to the heat load pulses applied in the early and late phases of wake per iod, while the response is significantly reduced to the pulse applied in th e supposed wake-maintenance zone. This bimodality of the response appears t o reflect the sensitivity of the HWSNs. In addition, the early pulse raises the immediate sleepiness rather than the nocturnal sleepiness, while the h eat load pulse applied in the later phase of waking period significantly ra ises the sleepiness during a nocturnal sleep. In simulations of sleep depri vation, the discontinuous relationship between recovery sleep length and de privation time is reproduced, where the critical sleep deprivation time at which the recovery sleep length jumps is extended as the losing rate increa ses. This is possibly due to the dissipation of the heat memory accumulated by the sleep deprivation. The simulation results here qualitatively reprod uce the experimental observations:or predict the intriguing phenomena of hu man circadian rhythms. Therefore, our model could provide a novel framework for investigating the relationship between thermoregulation and sleep cont rol processes.