SLEEP-DEPENDENT INFRASLOW RHYTHMS ARE EVOLUTIONARILY CONSERVED ACROSS REPTILES AND MAMMALS

Comparative studies are expanding our knowledge of the expression and evolution of sleep. However,
substantial gaps remain in our understanding of how and when sleep evolved in lizards, mammals, and birds,
the vertebrate classes comprising the clade Amniota. By recording brain activity in seven lizard species, as
well as in humans, rats, and pigeons, we demonstrate the conservation of an infraslow brain rhythm during
sleep. When eye movements, muscle tone, heart rate, and breathing rate are considered across lizards, in
addition to skin brightness in chameleons, we find a tight coupling of these measures with the infraslow
brain rhythm. Moreover, the similarly tight coupling of the infraslow rhythm with pulsatile changes in
cerebrovascular volume in bearded dragons and mice provides the most compelling evidence that the
infraslow rhythm is highly conserved across amniotes, suggesting that this rhythm was expressed in the
common ancestor. That the infraslow-cerebrovascular coupling in mice is specific to NREM sleep is
inconsistent with the notion that the infraslow rhythm in lizards is a “REM-like” state. Accordingly, in a
time of increased attention to sleep across vertebrates and invertebrates, we encourage greater caution when
considering the existence of specific mammal-defined sleep states in distantly related groups.