A nap can recalibrate homeostatic and associative synaptic plasticity in the human cortex

• Non-invasive indices in the human cortex of net synaptic strength are decreased after a nap. • Indices of inducibility of LTP-like plasticity are increased after a nap. • The findings show that even a brief daytime nap is sufficient to restore synaptic plasticity. • The findings provide a mechanistic links to the benefits of naps observed on behavioral levels. Nighttime sleep renormalizes net synaptic strength (homeostatic plasticity) and the inducibility of long-term potentiation (LTP)-like plasticity (associative plasticity) in the cortex. However, whether an afternoon nap is sufficient for this process remains to be characterized. Twenty healthy adults participated in a repeated measures sleep laboratory study with an adaptation and two experimental sessions – nap and wake (1:15–2:15 pm ). After the nap or wake session, non-invasive indices of net synaptic strength (indexed by transcranial magnetic stimulation, TMS-probed corticospinal excitability and wake EEG theta activity) and inducibility of LTP-like plasticity (indexed by TMS-induced motor evoked potentials, MEPs, following paired associative stimulation, PAS) were assessed. We observed indices of reduced net synaptic strength after sleep compared to wakefulness, evidenced by a higher TMS intensity needed to induce MEPs and reduced wake EEG theta activity. Additionally, we observed an increase in the inducibility of associative synaptic plasticity after sleep, as evidenced by a greater increase in TMS-induced MEPs in response to PAS. The study reinforces the restorative effect of sleep for homeostatic and associative synaptic plasticity in the human cortex and demonstrates that even a short nap can promote this process.