Sleep: Eye-Opener Highlights Sleep’s Organization Frederik D. Weber  Current Biology  Volume 28, Issue 5, Pages R217-R220 (March 2018) DOI: 10.1016/j.cub.2018.01.054 Copyright © 2018 Elsevier Ltd Terms and Conditions

Figure 1 Pupils predict sleep state and constrict to protect deeper offline periods. (A) Eye movements and pupil size reveal mice’s sleep stage: moving, and with pupils wide open during wake, but mostly constricted during rapid-eye-movement (REM) sleep, and not moving but periodically changing pupil size every minute in non-REM sleep. Pupil size and eye movements can automatically be inferred by (B) recordings of head-fixed mice that also have their eyes opened while sleeping (typically in the light phase of the day; top right, arrow indicates sleeping time). This simply requires an infra-red camera that records the eye while the retina is back-illuminated by an infra-red LED from inside the skull and highlights the pupil in the recording (infra-red back-illumination pupillometry, iBip [1]). (C) During non-REM, pupil size changes periodically on an infra-slow time scale about every minute. This is driven by parasympathetic influences on the constrictive ring muscle around the pupil. Infra-slow pupil constrictions align with activity in the brain and heart. Reduced pupil size potentially protects deeper ‘offline’ sleep-states characterized by increased memory consolidation processes and lower arousability (dark shade). This is marked by increased cortical sleep spindle activity associated with ripple activity in hippocampus (and posterior parietal cortex). The following and preceding ‘online’ states are indicated by pupil dilations that boost the eyes’ sensitivity to light and enhance the animal's chance to detect potential environmental threats. Concomitant heart-rate increases might anticipate a more aroused state necessary to react and evade such threats. Current Biology 2018 28, R217-R220DOI: (10.1016/j.cub.2018.01.054) Copyright © 2018 Elsevier Ltd Terms and Conditions