Prolonged Nightly Fasting in Breast Cancer Survivors

This study is being done to examine whether fasting for 13 hours every night is feasible and if it can help breast cancer survivors lose weight and improve their health.

* Previous studies have found that women who are overweight or obese when their breast cancer is found (diagnosed) have a greater risk of their breast cancer recurring. Recent research suggests that prolonged nighttime fasting (\>13 hours) may improve the risk of recurrence for breast cancer.
* This study will examine if fasting for 13 hours per night is doable for participants and will also study what the effect of fasting is on quality of life, mood, fatigue, body size, and markers of health in the blood.

This research study involves fasting (not eating any food or drinking fluids that contain calories) for 13 hours nightly for 12 weeks.

* It is expected that about 40 people will take part in this research study.
* Eligible participants will undergo baseline assessments prior to starting the intervention.

  * Baseline assessments include measurements of weight, height, quality of life, fatigue, mood, levels of physical activity, and blood markers.
  * Assessments will be repeated at the completion of the 12-weeks

This is a a Feasibility Study, which means this is the first time that investigators are examining prolonged nightly fasting and its effect on breast cancer survivors body size, blood markers, quality of life, emotional regulation, fatigue and level of physical activity.

Eligibility Criteria:
Inclusion Criteria:

* Participants must have a documented history of histologically confirmed invasive breast cancer.
* Participants with a history of stage I to III invasive breast cancer, and no current evidence of disease.
* A history of bilateral breast cancer is allowed provided the patient is currently disease free, with stage I to III disease on both sides.
* No evidence of distant metastatic disease or unresectable locally recurrent disease
* All adjuvant or neoadjuvant cytotoxic chemotherapy, radiation, and surgery for breast cancer must have been completed at least 6 month prior to registration. Except:

  * Adjuvant hormonal therapy is permitted. Must have been on for a minimum of 1 month.
  * Adjuvant trastuzumab, pertuzumab, TDM1, or neratinib for Her2 positive breast cancer is permitted Age ≥18 years.
* Participant must be female.
* Ability to understand and the willingness to sign a written informed consent document

Exclusion Criteria:

* Distant metastatic breast cancer (Stage IV breast cancer) or unresectable locally recurrent disease
* Participants with diabetes mellitus.
* Participants with a pre-existing eating disorder (anorexia nervosa, bulimia)
* Participants with a BMI\< 19kg/m2 or a weight loss of 5% in the last month or 10% in the last 3 months.
* Participants using weight loss medications at the time of study enrollment.
* Participants using oral steroids at the time of enrollment.
* Participants who are receiving any other investigational agents. Participants with uncontrolled intercurrent illness.
* Participants with psychiatric illness/social situations that would limit compliance with study requirements.
* Pregnant women are excluded from this study because the effects of prolonged fasting on the fetus are not known.