The purpose of this study is to evaluate the mechanisms through which Cognitive Behavioral Therapy for Insomnia (CBT-I) leads to improvements in sleep and sleep-related daytime outcomes, specifically as it relates to cortisol and the hypothalamic-pituitary-adrenal (HPA) axis, the body's primary stress response system. The primary research aims are: 1) To determine whether changes in cortisol functioning are related to improvements in sleep consolidation (4-week CBT-I), sleep duration (10-week CBT-I) or both, AND 2) to examine whether these improvements are more pronounced among individuals with insomnia and short sleep duration (compared to insomnia with 'normal' sleep duration).This study will recruit individuals with chronic insomnia: half with reported short sleep duration (less than 6 hours of sleep on an average night) and half with reported non-short sleep duration (6 or more hours per night). Participants will complete an in-lab assessment on four different occasions. During the first visit, they will confirm eligibility, complete an online survey, complete an in-lab stress task (with saliva samples), and provide a hair sample to measure cortisol (a hormone). They will then be sent home with a home sleep test as a final eligibility check (to rule out sleep disorders other than insomnia), an actigraphy watch to measure their sleep, and collection tubes to provide saliva samples to measure cortisol. We will then track the participants' sleep using online daily diaries and actigraphy for four week. Following the 4-week baseline period, participants will come in for a second in-lab visit (same procedures as the first visit) and then be randomized to receive either four or ten weeks of Cognitive Behavioral Therapy for Insomnia (CBT-I). CBT-I is a empirically supported behavioral treatment for insomnia and the first-line treatment for chronic insomnia. Sleep patterns and insomnia symptoms will continue to be monitored via diaries during the intervention phase. The third visit will take place once the participant has completed the intervention and the fourth visit will occur three months following the third visit (3 months post-treatment). The 3rd and 4th visits will again replicate the same procedures conducted during the initial lab visit.

Inclusion Criteria:

• At least 18-years-old
• Speaks English
• Meets DSM-5 criteria for Insomnia Disorder, determined through initial screener than verified during brief clinical interview (at intake)

Exclusion Criteria:

• Sleep disorders other than Insomnia
• Endocrine disorders
• Extreme sleep schedule (i.e. shift workers)
• Currently taking sleep medication
• Currently taking medication for endocrine dysfunction
• Currently taking medication that interferes with sleep or cortisol levels
• Excessive alcohol/substance use or AUD/SUD
• Serious mental illness
• Chronic medical conditions (which may be exacerbated by sleep restriction)
• Pregnant or nursing women
• Women in the peri-menopausal stage due to irregular menses and fluctuations in sex hormones.

De-identified participant data will be made available upon completion of the study and publication of the study results. These data will be made available online through a dedicated study site.

De-identified IPD will be made available indefinitely upon publication of the primary study results.