The goal of this clinical trial is to understand how level of adherence with time-restricted eating (TRE) predicts change in diurnal rhythms (as measured using the amplitude of diurnal peripheral clock gene expression), and how those changes predict lower mania and depressive symptoms, and downstream improvements in quality of life. The effects of diurnal amplitude of clock gene expression is expected to remain significant when controlling for change in glucose tolerance and inflammation. Participants will be enrolled who are already receiving medication treatment for bipolar disorder. Participants will complete daily measures of eating, sleep and mood for two weeks, and then will be assigned to follow TRE for eight weeks. Symptoms and Quality of Life will be measured at baseline and during and after the food plan.

This is a single-arm trial to examine the effects of time-restricted eating on change in diurnal rhythms, manic and depressive symptoms, and quality of life. In time-restricted eating (TRE), participants will be asked to limit their food intake to a period of 10 hours per day. TRE will be an addition to standard medication approaches in bipolar disorder. Participants who are receiving medical treatment for bipolar disorder and who report at least some sleep or circadian problems will complete baseline measures and then will be asked to follow TRE for 8 weeks, and then will complete measures of symptoms, Quality of Life, and possible treatment mechanisms at the mid-point of treatment, the end of treatment, and at 3 months after the intervention. If successful, this work will help understand key mechanisms through which TRE provides benefits for those with BD.

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)

Measures, data, and analysis scripts will be shared through Open Science Foundation.

De-identified data will be shared within one year after data collection ends.

Data will be available publically through Open Science Foundation upon request. Wellcome Trust guidance for data sharing will be followed.

Outcomes will be assessed by interviewers. Where possible, interviewers will be unaware of the treatment condition (interviewers will evaluate symptoms for more than one study, allowing us to keep them unaware of treatment condition).