Across their medical careers, Cleveland Clinic chief wellness officer and New York Times best-selling author Michael Roizen, M.D., and Johns Hopkins faculty member and medical director for The Dr. Oz Show Michael Crupain, M.D., have long emphasized the therapeutic value of food. In their book What to Eat When (National Geographic, 2019), the authors explore the science behind food as medicine and explain how not only the foods you choose but the timing of those choices can influence your health, energy, sexual health, weight, mood and the way you age.
What alerted you to the importance of adding time as a variable to healthy eating?
Crupain: The nutrition world has spent decades focusing on the “what” of eating—comparing fruits with fries, nuts with chips and so on. Far less attention has gone to the “when.” Mike and I serve on the board of HealthCorps, a health-education charity, and a few years back I suggested collaborating on a project at the Cleveland Clinic. He replied, “Let’s write a book together,” and that’s how this work began.
Roizen: The evidence tying timing to health began surfacing about ten years ago in animal studies and more recently in human research. As we reviewed the literature and our clinical experience, it became clear that timing of meals matters substantially for longevity and quality of life. The body’s response to food changes across the day, and the growing data convinced us this was important enough to share widely.
Describe the “When Way” of eating.
Roizen: Eating in alignment with your circadian rhythm—the body’s internal clock that regulates sleep and wake cycles—gives you measurable advantages. When you time food to match those natural rhythms, you often see improvements in energy, sleep quality, metabolic health and weight management.
Crupain: We boiled the approach down to four practical guidelines for the When Way: 1) Eat primarily during daylight hours—sync your meals with your circadian rhythm so your body performs the right functions at the right times; 2) Eat more earlier and less later—aim for roughly 75 percent of your daily calories to fall before mid-afternoon; 3) Keep your eating schedule consistent from day to day to help your body anticipate and adapt; and 4) Be flexible about traditional meal roles—there’s no rule that says eggs are only for breakfast or that salad is only for dinner. Let food fit the time, not rigid food stereotypes.
You write that “our body’s internal environment is always changing—and how you feed it during those changes matters.” Is that why you came up with the “What’s the Situation” section for what to eat in different life scenarios, such as when you’re stressed and hangry, when you can’t sleep, when you have a job interview?
Crupain: Exactly. Life presents many different circumstances, and a one-size-fits-all eating plan won’t address everything. The “What’s the Situation” sections came from clinical experience, conversations with patients and familiar day-to-day challenges. They cover a broad spectrum—some scenarios are urgent or serious, others are routine—but all reflect common moments when adjusting what and when you eat can make a real difference.
Roizen: Patients often ask very specific questions: what to eat when stressed, how to handle travel or shift changes, what to choose before an important meeting or a romantic evening. As clinicians and scientists, we paired those real-world questions with data where it exists. The result is practical guidance grounded in both evidence and experience.
You offer a 31-day plan. After one gets through that, what’s next?
Crupain: The 31-day plan is designed as a transition, not a short-term fix. It helps people shift gradually to a timing-focused way of eating, because small, progressive changes are more sustainable biologically and behaviorally. The first half of the month emphasizes meal timing and portion adjustments; the second half places more focus on food quality—emphasizing plant-based choices and reducing added sugars. Ideally, by day 31 you’ll have established new habits that can be maintained long term.
If you’re not fully comfortable or ready after 31 days, that’s fine—the plan is flexible. Some people repeat parts of it, take longer to transition, or adopt selected elements that fit their lifestyle. The goal is sustainable improvement: align your eating with your daily rhythm, make the majority of calories earlier in the day, emphasize whole, plant-forward foods and reduce refined sugars. Those changes can be continued indefinitely and tailored to individual needs and routines.