Inclusion Criteria:
* Meets diagnostic criteria for bipolar I disorder or bipolar II disorder (but not cyclothymia, bipolar disorder Not otherwise specified or bipolar disorder due to another medical condition) assessed by the Diagnostic Interview for Anxiety, Mood, and Obsessive-compulsive and Related Neuropsychiatric Disorders \[DIAMOND\])
* current sleep (insomnia, hypersomnolence) or circadian sleep-wake (delayed phase, advanced phase, irregular sleep-wake, non-24-hour sleep-wake-type) concerns indicated by endorsement of at least some sleep or circadian-related impairment across the screening self-reports or interview
* Living in an English-speaking country (and one that the researchers have expertise in research procedures and diet)
* Has been speaking English for at least 10 years, speaks English in the home, or certifies that they are able to understand English well for the study and demonstrates this during the screening interview.
* Receiving medical care for bipolar disorder (referrals will be provided for those who would like to begin care)
* Mood-stabilizing medication regimens stable for at least one month
* \< 5 kg weight change in the past 3 months
* Currently eating ≥ 12 hours per day at least twice per week
* Able to operate the camera function and respond to web-based surveys by phone (loaner phones will be provided as needed)
* Not engaged in current shift work or have other responsibilities such as providing care that would chronically disrupt their sleep (i.e., \> 3 h between 22:00 and 05:00 h for at least 1 day/week)
* Able to complete 7 days of dietary logs adequately (e.g., at least 2 entries per day, covering at least a 5-hour eating window) during the baseline period
* Able to complete screening and baseline questionnaires adequately (e.g., not failing more than 1 attention check item with instructed responding; responding to standard multiple-choice items in a mean of \< 2 seconds per item). Where individuals respond to more than 14 items in a row with the same response, their answers will be manually reviewed for possible invalidity.
Exclusion criteria include the following:
* Current episode of depression, hypomania or mania, or psychosis (assessed by the DIAMOND), Participants with acute mood disorder episodes will be encouraged to seek treatment and to consider the study when symptoms have remitted.
* Eating disorder diagnosis (by self-report of treatment or diagnosis at any point during their life, Short Eating Disorder Examination Questionnaire (EDE-QS) scores above clinical concern thresholds for eating disorders, or DIAMOND interview of symptoms during adulthood)
* Past 3-month alcohol use disorder or substance use disorder (assessed by DIAMOND)
* Active suicidal ideation coupled with plan, intent or attempt history as assessed by Columbia Suicide Severity Rating Scale
* Conditions that would interfere with ability to take part in the intervention, including pregnancy, breastfeeding, uncorrected hypo or hyperthyroidism, gastrointestinal conditions impairing nutrient absorption
* Conditions that would confound immune or other study measures, such as HIV, AIDS, lupus, or multiple sclerosis
* Medications contraindicated for fasting: clozapine, glucose-lowering medications, diabetes-related injections, medications requiring food early morning or late evening, corticosteroids; Glucagon-like peptide-1 (GLP-1) agonists will not be an exclusion criteria if weight stabilized
* Cognitive deficits as noted during the initial interview or as indicated by low performance on the Orientation Memory Concentration Test- Short Version (weighted score \< 20)
