Fig. 1.

Historical and current understanding of the role astrocytes in sleep. Top: Cajal postulated that astrocytes retract their processes during wakefulness to allow normal synaptic transmission. During sleep, he reasoned, astrocyte processes invade the synapse to block synaptic transmission. Bottom: Recent findings revise this model to include astrocyte processes covering the synapse, presumably to enhance glutamate clearance during wake and retracting during sleep. This correlates with changes in gene expression in astrocytes (with ~400 transcripts up-regulated during wake and ~50 during sleep) and could relate to more glutamate spillover and increased glymphatic flow during sleep. Previous findings also showed dendritic spines grow with time awake (and greater sleep debt; reviewed in [3]). The model thus posits that changes in glial gene expression, morphology, and physiology may modulate synaptic transmission to promote sleep