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

BACKGROUND 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.

METHODS 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.

RESULTS 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.

CONCLUSIONS 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.