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Feel Groggy & Tired When Intermittent Fasting? This Timing Issue May Be The Reason Why
Most people who try intermittent fasting shrink their eating window by holding off on food until 11 a.m. But according to nutrition researcher Rachele Pojednic, PhD, this popular approach doesn't get the science exactly right, and that's the problem.
"There actually is a fair amount of data on fasting," Pojednic said on the mindbodygreen podcast. In the research world, the practice is referred to as time-restricted eating: eating within a specific window, typically around six to eight hours in the studies.
She says issue isn't this concept, but how most people apply it.
Why skipping breakfast works at first
Most of the people who skip breakfast while intermittent fasting are looking to lose weight. And early on, Pojednic says the strategy does deliver, but only because "they've just cut 500 calories out of their diet." Calories in versus calories out is the simplest way to determine whether your body maintains, loses, or gains weight, and most people decrease calories when they begin intermittent fasting. But that effect only lasts so long.
But eventually, people following this eating pattern begin to feel tired or realize they are not sleeping well. Women especially feel this, which tracks with what researchers have found when looking at how women's bodies respond differently to popular fasting protocols compared to the men's data those protocols were often built on.
When people who are intermittent fasting by skipping breakfast begin feeling fatigued, Pojednic says two things are probably happening. First, they may simply be eating too little, and their body responds by shutting things down, which produce fatigue.
The second problem has to do with skipping that morning meal, wihch Pojednic says, "actually seems to be the opposite of what the majority of the data is showing."
Instead, close your eating window before bed
Pojednic explained that the research on time-restricted eating has actually tested the effects of when you stop eating at night, not on how long you can go without food in the morning. Specifically, Pojednic suggests that finishing your last bite somewhere between two and four hours before you go to sleep.
A 2026 analysis1 of 3,463 adults (51% of them women), points in the same direction. People with a longer gap between their last meal and bedtime had better sleep quality and better mental and physical functioning, while those with longer daily eating windows slept worse.
"It will help you sleep better. It improves your circadian rhythm. It also improves things like visceral fat," Pojednic said. Circadian rhythm refers to your body's internal 24-hour clock, and visceral fat is the deeper fat stored around your organs. She added that there's also some data showing improvements in blood pressure and triglycerides, a type of fat found in your blood.
She says this is because, when you stop eating earlier, your body has a longer period to rest and digest overnight. "I think any of us can probably intuit that you don't wanna eat a super large meal right before you go to bed," Pojednic postures.
This logically makes sense, but the evidence on visceral fat and triglycerides is more mixed. A 2025 trial in Nature Medicine2 found that time-restricted eating didn't reduce visceral fat compared with usual care alone, and a 2025 meta-analysis of studies in women3 found no significant effect on visceral fat. A 2025 trial comparing early and late eating windows found no difference in triglycerides between the groups.
A fasting rhythm that starts with breakfast
Pojednic laid out what a better daily pattern looks like. You wake up, eat something fairly early to fuel up for the day, then close the kitchen a few hours before bed. This schedule works with your body's innate schedule rather than against it.
When you eat during the day can matter as much as the length of the window itself, and a 2022 randomized trial4 in young adults with overweight or obesity tested the windows head-to-head.
For eight weeks, one group ate between 7 a.m. and 1 p.m., another ate between noon and 6 p.m., and a third had no time limit. Both timed groups lost a similar amount of weight. Only the early eaters, though, saw lower systolic blood pressure (the top number), average blood sugar, fasting insulin, and insulin resistance, which is when your cells stop responding well to insulin. The later group saw a drop only in leptin, a hormone that helps regulate appetite.
With intermittent fasting, you're still eating fewer calories, so weight loss can follow, and the benefits stack from there. "You're sleeping better, so you don't feel really tired, and your overall metabolism is improving, so you have more energy throughout the day," Pojednic says. Shifting the time-restricted eating window earlier will prevent people from feeling crummy on the backend.
The takeaway
The most effective fast may be the one that happens while you sleep, not the one that leaves you running on empty all morning. Ending your last meal a few hours before bed gives your body the overnight downtime the research points to, while breakfast keeps your energy steady.
And none of this is requires complicated protocol to test out. If you want to try intermittent fasting, pick a cutoff time two to four hours before you plan to turn in, eat a real breakfast the next morning, and see how you feel.