Evidence of Sleep Cycle Analogous to Vertebrate SWS/REM Alternation in the Octopus

Abstract

The combination of body quiescence and skin pattern changes displayed periodically by Sepia officinalis, as well as the high threshold for arousal upon stimulation and increased brain activity in quiescent Octopus vulgaris, suggest the existence of a state akin to rapid-eye-movement sleep (REM) in cephalopods. Would be possible that cephalopods, as amniotes, sleep through the alternation of distinct quiescent states, analogous to slow-wave sleep (SWS) and REM? To investigate this possibility, we video-recorded 4 adult specimens of Octopus insularis and quantified in detail their behavioral states and transitions. We also performed sensory stimulation to measure reaction times across all behavioral states. Two distinct quiescent states occurred in tandem, both quite unresponsive to stimulation (median reaction time: 50 s). The first state was SWS-like, characterized by uniformly pale skin, closed pupils, and long episode durations (median 415.2 s). The second state was REM-like, characterized by dynamic skin patterns, rapid eye movements and short episode durations (median 40.8 s). The REM-like state was periodic (60% of all recurrences between 26.85-39.15 min) and occurred mostly after the SWS-like state (82% of all transitions). These observations support the existence in cephalopods of an ultradian sleep cycle analogous to the SWS/REM alternation.