Wavelength-dependent sleep state manipulation using light pulses in Pogona vitticeps.

Slow-wave (SW) and rapid eye movement (REM) sleep states are defining features of mammalian sleep. Controlling them is key to unraveling their function and developing therapeutic strategies for sleep disorders. We investigated sleep state manipulation using light in Pogona vitticeps, a lizard exhibiting highly structured, periodic transitions between sleep states. We demonstrate state entrainment using light pulses, overriding the natural sleep state rhythm, and revealing a mechanism for non-invasive modulation of sleep architecture. External light pulses transiently increase spiking followed by a transition to a SW-like state, characterized by elevated δ/β oscillation power, increased sharp-wave activity, and reduced baseline spiking. This state persists after stimulation offset, and is followed by REM-like activity. Notably, state modulation was wavelength-dependent: white and red lights produced robust entrainment whereas blue and green lights yielded weaker modulation. Replacing external illumination with direct intracranial light delivered via an optic fiber, at intensities commonly used in optogenetics, resulted in similar entrainment and wavelength-dependence, but perturbed natural sleep, as indicated by increased movement and reduced δ/β power. Our findings identify light pulses as a potent strategy for controlling sleep-state dynamics and dissecting the mechanisms and evolutionary origins of sleep-state transitions and their modulation by external light.