TY - JOUR T1 - A Path to Sleep is Through the Eye JF - eneuro JO - eneuro DO - 10.1523/ENEURO.0069-14.2015 SP - ENEURO.0069-14.2015 AU - Lawrence P. Morin Y1 - 2015/03/05 UR - http://www.eneuro.org/content/early/2015/03/04/ENEURO.0069-14.2015.abstract N2 - Light has long been known to modulate sleep, but recent discoveries support its use as an effective nocturnal stimulus for eliciting sleep in certain rodents. “Photosomnolence” is mediated by classical and ganglion cell photoreceptors and occurs despite the ongoing high levels of locomotion at the time of stimulus onset. Brief photic stimuli trigger rapid locomotor suppression, sleep and a large drop in core body temperature (Tc; Phase 1), followed by a relatively fixed duration interval of sleep (Phase 2) and recovery (Phase 3) to pre-sleep activity levels. Additional light can lengthen Phase 2. Potential retinal pathways through which the sleep system might be light-activated are described and the potential roles of orexin (hypocretin) and melanin-concentrating hormone are discussed. The visual input route is a practical avenue to follow in pursuit of the neural circuitry and mechanisms governing sleep and arousal in small nocturnal mammals and the organizational principles may be similar in diurnal humans. Photosomnolence studies are likely to be particularly advantageous because the timing of sleep is largely under experimenter control. Sleep can now be effectively studied using uncomplicated, non-intrusive methods with behavior evaluation software tools; surgery for EEG electrode placement is avoidable. The research protocol for light-induced sleep is easily implemented and useful for assessing the effects of experimental manipulations on the sleep induction pathway. Moreover, the experimental designs and associated results benefit from a substantial amount of existing neuroanatomical and pharmacological literature that provides a solid framework guiding the conduct and interpretation of future investigations. Significant Statement: Sleep is expressed as a circadian rhythm and the two phenomena exist in a poorly understood relationship. Light affects each, simultaneously influencing rhythm phase and rapidly inducing sleep. Here, multiple effects of light on sleep are discussed, as are the possible anatomical pathways by which photic input might reach brain stem sleep regulating nuclei. Emphasis is placed on a simple procedure by which a brief light stimulus rapidly induces sleep. Studies of the photic input pathway through which light elicits sleep, combined with modern soft- and hardware tools enabling studies of behavioral sleep absent the EEG and neurosurgery, are likely to provide a very productive avenue for probing the mechanisms of sleep onset and regulation. ER -