The Role of mGluR5 in CBT-I

Inclusion Criteria: * capacity to provide informed consent, * at least 18 years of age * good physical health and absence of significant medical conditions, * diagnosis of major depressive disorder (MDD) with current major depressive episode as per DSM-5 criteria * Montgomery-Åsberg Depression Rating Scale (MADRS) score \> 29 (at least moderate depression), * uniform range of sleep latencies up to 128 minutes. Exclusion Criteria: * currently taking effective antidepressants, * lifetime history of psychosis, * drug or alcohol dependence in the last 6 months or abuse in the last 2 months * unstable medical condition (i.e., condition not adequately stabilized for ≥ 3 months) * nicotine use within 1 year * currently on medication known to affect glutamate, * sleep disorders, other than insomnia, such as sleep apnea, * recent (within 1 year) regular night shift work (or rotating shift work) or recent (within 3 months) travel across more than one time zone, or plans for this work or travel during the study period, * use of medication or substances that affect sleep, for example, ingestion of more \> 600 mg of caffeine per day, * PET or MRI contraindications, including pregnancy or currently breastfeeding