Sleep in crayfish: Relationships between brain electrical activity and autonomic variables

Sleep in mammals is a dynamic process involving different stages of sleep intensity, which are associated with measurable changes in brain electrical activity. In this animal group two types of sleep have been described: rapid eye movement (REM) sleep and no-REM (NREM) sleep, each one has a specific brain electrical activity and is accompanied by variations in cardiac and respiratory frequencies. Sleep has been demonstrated only in a handful of invertebrates, and evidence for different sleep stages remains elusive. Crayfish sleep fulfills behavioral and electrophysiological criteria defined for mammals. Although no autonomic anatomical structures have been described in this animal, it is known that heart and respiratory frequencies are modified by diverse changes in its environment during wakefulness. Nonetheless, we do not know if this animal has sleep phases and what is the pattern of cardiorespiratory activity during sleep. Here, we studied physiological time series from crayfish based on brain and cardiorespiratory activity recordings in order to search for sleep phases. We used the wavelet transform to analyze brain and cardiorespiratory electrical activity. Our results show that the depth of sleep, measured as the power of electroencephalographic activity, changes over time and is accompanied by oscillations in cardiorespiratory signal amplitude and power. We propose that there are several phases of sleep in crayfish and that this invertebrate is a good biological model in order to achieve a better understanding of sleep.