  A biophysical model of arousal dynamics is used to study mechanisms of
synchronisation and loss of synchrony among the three key oscillators
controlling sleep-wake cycles: the circadian, homeostatic, and light
oscillators. Synchronisation of these rhythms promotes sleep and brain
clearance and is critical for human health. Conversely, their desynchrony is
linked to impaired performance and disease. We find that the default state of
the model corresponds to the endogenous homeostatic period that is far from ~24
h of the circadian and light-dark cycles. Combined action of light and
circadian oscillator on the homeostatic rhythm is required to achieve the
typical sleep-wake pattern of healthy people. Change of homeostatic clearance
is found to induce two types of desynchronisation: (i) fast clearance rates
desynchronise the homeostatic oscillator from the circadian, while the
circadian rhythm remains entrained to light, and (ii) slow clearance maintains
synchronisation between the homeostatic and circadian oscillators, but the
period is different from that of the light-dark cycle. Between these regimes,
all three rhythms are synchronised under the studied conditions. The system is
highly sensitive to external inputs to the neuronal populations of the
sleep-wake switch, which can lead to complete loss of sleep. The model shows
that loss of synchronisation can be caused by changes in the homeostatic
clearance rate or external input to the neuronal populations of the sleep-wake
switch. This has implications for understanding individual variability in
sleep-wake patterns and in mechanisms of sleep and circadian disorders.
