The goal of this clinical trial is to learn if restricting the time of eating to allow for prolonged fasting at night may reduce sleep disturbances, cognitive decay, and pathology in patients diagnosed with Mild Cognitive Impairment (MCI) or early to moderate Alzheimer's disease (AD). It will also learn about the feasibility of practicing 14 h of nightly fasting in this group of older adults. The main questions it aims to answer are: * Does prolonged nightly fasting of 14 h can reduce markers of AD pathology and aging and reduce cognitive and sleep alterations in MCI and AD patients? * Can patients with MCI and early /moderate AD sustain time-restricted eating for 3 to 6 months? Researchers will compare participants who fast for 14 h per night during 3 months to those who fast for less than 12 h/night. Researchers will also compare participants that fast for 3 months to those who fast during 6 months, to determine the effective duration of the intervention. Finally, researchers will evaluate whether following the time-restricted eating diet alongside a partner actively following the same diet, will increase adherence to the protocol compared to subjects that fast alone. Participants will: * Fast for 14 h a night (stop eating at 8 pm and start eating the following morning at 10 am) for 3 or 6 months * Visit the clinic three times (at the beginning of the study, 6 and 12 months later) * Provide blood samples and take a cognitive test during clinic visits * Keep a diary (or use an app on a smart phone) to record time of eating * Wear an activity tracker watch