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