Characterising Sleep Spindles in Sheep

Abstract Sleep spindles are distinctive transient patterns of brain activity that typically occur during non-rapid eye movement (NREM) sleep in humans and other mammals. Thought to be important for the consolidation of learning, they may also be useful for indicating the progression of aging and neurodegenerative diseases. The aim of this study was to characterise sleep spindles in sheep (Ovis aries). We recorded electroencephalographs (EEG) wirelessly from 6 sheep over a continuous period containing two nights and a day. We detected and characterised spindles using an automated algorithm. We found that sheep sleep spindles fell within the classical range seen in humans (10- 16 Hz), but we did not see a further separation into fast and slow bands. Spindles were detected predominantly during NREM sleep. Spindle characteristics (frequency, duration, density, topography) varied between individuals, but were similar within individuals between nights. Spindles that occurred during NREM sleep in daytime were indistinguishable from those found during NREM sleep at night. Surprisingly, we also detected numerous spindle-like events during unequivocal periods of wake during the day. These events were mainly local (detected at single sites) and their characteristics differed from spindles detected during sleep. These ‘wake spindles’ are likely to be events that are commonly categorised as ‘spontaneous alpha activity’ during wake. We speculate that wake and sleep spindles are generated via different mechanisms, and that wake spindles play a role in cognitive processes that occur during the daytime. Statement of Significance Sleep spindles provide an indication of brain health and function. In this study we characterise sleep spindles in sheep (Ovis aries) for the first time. We found that sleep spindles in sheep are similar to those found in humans in many respects (such as density, duration and frequency) and occurred mainly during NREM sleep. Interestingly however, we also saw spindles during wake in the day. Spindles detected during wake were characteristically distinct from those occurring during sleep. We suggest that wake and sleep spindles are generated via different mechanisms and may have different functional roles. Wake spindles may be a component of cognitive processes that occur during the daytime, such as memory retrieval and attention.