Exploring the link between waking gamma and sleep delta power in healthy volunteers and individuals with treatment-resistant depression.

Background: Sleep disruptions are a core feature of both major depressive disorder and treatment-resistant depression (TRD), which is defined by persistent symptoms despite multiple treatment efforts. In addition, disruptions in wakeful gamma power and sleep-related delta power have been observed in individuals with TRD. This study explored the association between gamma oscillations (30-100 Hz) occurring during wakefulness and delta power (0.5-4 Hz) in non-rapid eye movement (NREM) sleep-both of which have separately been implicated in plasticity-in healthy volunteers (HVs) and individuals with TRD. Specifically, the study explored whether a relationship exists between daytime wake gamma power and sleep NREM delta power in HVs and those with TRD.

Methods: Brain activity was measured via electroencephalography (night-time) and magnetoencephalography (daytime resting state) in 23 HVs (9M/14F; 20-56 yrs) and 40 medication-free TRD participants (20 M/20F; 18-63 yrs).

Results: In HVs, NREM episode (NREM1) sleep delta power correlated with daytime wake gamma power (r = 0.417; p = 0.04). This correlation was absent in TRD participants (r = 0.108; p = 0.50).

Limitations: The sample size of the HVs (n = 23) was smaller than the TRD participants (n = 40), and the measurement order for wake gamma power and sleep delta power varied.

Conclusions: These findings identify a possible link between daytime wake gamma power and NREM1 sleep delta power in HVs, supporting an association between gamma and delta power in sleep homeostasis. The lack of such a relationship in medication-free individuals with TRD suggests disrupted synaptic homeostasis that may contribute to impaired synaptic plasticity in TRD. Clinical Trials Identifiers: NCT00088699 and NCT01204918.