If you’ve ever felt like your body holds onto weight despite eating “healthy,” or if late-night cravings sabotage your progress, there’s a scientific reason behind it. Our bodies are naturally more insulin resistant at night, meaning that food consumed in the evening—especially carbohydrates—can have a greater negative impact on metabolism, blood sugar regulation, and weight loss.
In her book Fast Like a Girl, Dr. Mindy Pelz explains how our circadian rhythms, hormones, and fasting cycles influence insulin sensitivity. Understanding these factors can help us make better choices to improve metabolic health and reverse insulin resistance.
Why Are We More Insulin Resistant at Night?
Our bodies operate on a 24-hour circadian rhythm, which influences everything from sleep to digestion. Research shows that insulin sensitivity follows this natural rhythm—it is highest in the morning and decreases as the day progresses. Here’s why:
1. Circadian Rhythm and Insulin Sensitivity
- Our metabolism is wired to be most active during daylight hours. As the sun sets, melatonin increases to prepare the body for rest, which in turn reduces insulin’s effectiveness.
- A 2015 study published in Current Biology found that eating out of sync with circadian rhythms—such as consuming meals late at night—reduces glucose tolerance and increases insulin resistance (Morris et al., 2015).
- According to Fast Like a Girl, Dr. Pelz explains that nighttime eating can spike glucose levels significantly higher than the same meal eaten earlier in the day.
2. Cortisol and Blood Sugar Regulation
- Cortisol, the stress hormone, follows a natural cycle. It peaks in the morning to help wake us up and tapers off in the evening. However, when we eat late at night, cortisol can rise again, increasing blood sugar and reducing insulin’s effectiveness.
- Elevated nighttime cortisol levels have been linked to poor sleep quality, weight gain, and worsening insulin resistance (Scheer et al., 2009).
3. Melatonin’s Role in Insulin Resistance
- Melatonin, the hormone that signals sleep, directly suppresses insulin secretion. This means that eating close to bedtime causes blood sugar to remain elevated longer than it would during the day.
- A study published in Diabetes Care found that late-night meals resulted in significantly higher blood sugar levels compared to identical meals eaten earlier, due to melatonin’s impact on pancreatic function (Garaulet et al., 2020).
How Late-Night Snacking Sabotages Weight Loss and Metabolic Health
Given our body’s natural decrease in insulin sensitivity at night, consuming food—especially carbohydrates and processed foods—during this time has major consequences:
1. Increases Fat Storage
- Since insulin isn’t as effective at night, glucose remains in the bloodstream longer, leading to increased fat storage.
- A 2017 study published in Obesity showed that eating late at night significantly increased body fat compared to eating the same calories earlier in the day (McHill et al., 2017).
2. Leads to Higher Fasting Blood Sugar
- Late-night eating can cause elevated glucose levels that persist into the next morning, making it harder to stabilize blood sugar throughout the day.
- Research published in The Journal of Clinical Endocrinology & Metabolism found that participants who ate later in the evening had higher fasting blood sugar levels and greater insulin resistance than those who ate earlier (Qian et al., 2019).
3. Disrupts Growth Hormone and Fat Burning
- Our bodies enter repair and fat-burning mode at night, facilitated by growth hormone. However, eating late disrupts this process because the body prioritizes digestion over cellular repair and fat metabolism.
- A study in Cell Metabolism found that restricting food intake to earlier in the day improved metabolic health and fat oxidation, independent of total calorie intake (Sutton et al., 2018).
4. Triggers Late-Night Cravings and Overeating
- Eating late, especially processed or high-carb foods, spikes blood sugar, which then crashes a few hours later—leading to more hunger, cravings, and overeating the next day.
- Research suggests that late-night eaters tend to consume more calories overall, often due to dysregulated hunger hormones (Gill & Panda, 2015).
What Can You Do to Support Insulin Sensitivity and Metabolic Health?
1. Eat During Your Natural Insulin-Sensitive Window
- Dr. Pelz recommends eating your largest meals earlier in the day when insulin is most effective.
- Aiming for an early dinner (before 6-7 PM) can significantly improve metabolic function.
2. Try Intermittent Fasting
- Time-restricted eating, such as a 12-16 hour fasting window, allows insulin levels to reset overnight and supports fat burning.
- Fast Like a Girl emphasizes that fasting works best when aligned with a woman’s hormonal cycles, so syncing fasting with your body’s needs is key.
3. Prioritize Protein and Healthy Fats at Night
- If you must eat late, opt for protein and healthy fats instead of carbs to prevent glucose spikes.
- Examples: Bone broth, nuts, seeds, eggs, or a protein-based snack can keep blood sugar stable.
4. Optimize Sleep to Regulate Blood Sugar
- Poor sleep increases insulin resistance. Aim for 7-9 hours of high-quality sleep to support metabolic function.
- Reduce blue light exposure from screens before bed to support melatonin production.
Understanding that our bodies are naturally more insulin resistant at night can help us make better dietary choices for metabolic health and weight loss. Dr. Mindy Pelz’s insights in Fast Like a Girl confirm that avoiding late-night snacking, eating within our insulin-sensitive window, and prioritizing sleep can all play a crucial role in reversing insulin resistance and improving overall health.
By aligning your eating habits with your body’s natural rhythms, you’ll not only support insulin function but also enhance your ability to burn fat, stabilize blood sugar, and feel more energized throughout the day.
Want to take control of your metabolic health? Try shifting your eating window earlier and track how your body responds!