Is it Normal to Feel Sleepy When Dieting?

November 17, 2025 •

4 min read

According to a CNN report, restricting calories too dramatically can lead to a state of low energy or fuel deficiency, which is why many people wonder: is it normal to feel sleepy when dieting? This common side effect is often a sign that your body is adapting to a lower energy intake, signaling that some adjustments may be needed to maintain your energy levels.

Quick Summary

Feeling tired during a diet is a common side effect of calorie restriction and can be influenced by metabolic changes, nutrient deficiencies, and dehydration. Several strategies can help manage low energy, including adjusting calorie intake, rebalancing macronutrients, and improving sleep habits. Addressing this issue is key to sustaining a healthy weight loss plan.

Key Points

Calorie Deficit: Significant or rapid calorie reduction triggers your body's survival mode, slowing metabolism and causing fatigue to conserve energy.
Nutrient Deficiencies: Lack of essential nutrients like iron, B12, protein, and complex carbohydrates can lead to low energy and extreme tiredness.
Hydration is Key: Dehydration is a common cause of sluggishness and fatigue during a diet; increasing water intake can improve energy levels.
Balance Macronutrients: A diet balanced in protein, healthy fats, and fiber-rich carbs provides sustained energy and prevents blood sugar crashes associated with refined sugars.
Mindful Exercise: While beneficial, excessive exercise combined with low calories can cause burnout. Moderate intensity and sufficient recovery are crucial for maintaining energy.
Listen to Your Body: Track your energy levels and adjust your diet or exercise as needed. If severe fatigue persists, consult a healthcare professional to rule out underlying issues.
Quality Sleep Matters: Poor sleep quality, often a side effect of dieting, can worsen fatigue. Prioritizing 7-9 hours of restful sleep is vital for recovery and energy.

The Connection Between Caloric Deficit and Fatigue

Creating a calorie deficit is the fundamental principle of weight loss, but it's also the primary reason many people experience fatigue. When you consume fewer calories than your body needs to maintain its current weight, your body enters a state of negative energy balance. While this is necessary for shedding pounds, a significant or rapid drop in energy intake can be interpreted by your body as a famine. In response, it attempts to conserve energy by slowing down your metabolism and making you feel tired and lethargic. The larger the deficit, the more pronounced this effect is likely to be.

Impact on Metabolism

When your body perceives a calorie shortage, it can decrease the activity of the thyroid gland, an effect sometimes referred to as 'pseudo-hypothyroidism'. This reduction in thyroid function lowers your overall energy expenditure, which helps preserve your stored energy but also results in decreased energy levels and a reduced desire to move. In the short term, this can help you lose weight, but if not managed correctly, it can lead to burnout and make it harder to stick to your diet long-term.

The Role of Nutritional Deficiencies

Overly restrictive diets, such as those that eliminate entire food groups, can lead to nutrient deficiencies that directly impact your energy levels. Your body needs a complex array of macronutrients (carbohydrates, proteins, and fats) and micronutrients (vitamins and minerals) to function optimally.

Inadequate Carbohydrate Intake: Carbohydrates are the body's primary source of energy, converted into glucose to fuel daily activities. Drastically cutting carbs, especially fiber-rich complex carbs, can lead to low blood sugar and the associated symptoms of fatigue and irritability.
Low Protein Intake: Protein is vital for muscle repair and growth. Not consuming enough protein can lead to muscle loss and decreased strength, leaving you feeling weak and tired.
Micronutrient Deficiencies: Crucial vitamins and minerals like iron, B12, and Vitamin D are essential for energy production. A shortage of these can lead to anemia and severe fatigue.

Comparison of Dieting Strategies and Energy Levels


Dieting Strategy	Energy Level Effect	Primary Cause of Fatigue	Potential Solution
Extreme Calorie Restriction	High likelihood of significant fatigue and sluggishness.	The body's starvation response, slowing metabolism to conserve energy.	Moderate the calorie deficit to a more sustainable level.
Low-Carbohydrate Diet	Fatigue, especially during the initial phase (keto flu).	Depletion of glycogen stores, as the body transitions to using fat for fuel.	Incorporate healthy, complex carbohydrates and stay hydrated.
High-Protein, Low-Fat	Can cause fatigue if healthy fats and other nutrients are overlooked.	Lack of healthy fats for cellular function and absorption of fat-soluble vitamins.	Ensure a balanced intake of healthy fats, such as from avocados and nuts.
Balanced, Moderate Deficit	Minimal initial fatigue, with energy improving over time.	Slight adjustment period as the body adapts to a moderate calorie reduction.	Stay consistent with nutrition, stay hydrated, and prioritize sleep.

Lifestyle Factors Aggravating Diet Fatigue

Dieting doesn't occur in a vacuum, and other lifestyle factors can amplify feelings of tiredness.

Poor Sleep Quality: The stress of dieting and changes in eating patterns can disrupt sleep. Ironically, sleep deprivation is also linked to weight gain, creating a vicious cycle. Ensuring you get 7-9 hours of quality sleep per night is crucial for combating daytime sleepiness.
Dehydration: Your body's functions, including energy production, rely heavily on proper hydration. When dieting, especially with increased exercise, it's easy to become dehydrated, which can manifest as fatigue and sluggishness.
Increased Exercise: Combining a new or intensified exercise routine with a calorie-restricted diet can lead to burnout. Your body needs sufficient calories and recovery time to repair muscles and restore glycogen stores. Gradually increasing your workout intensity and prioritizing rest days is key to avoiding exhaustion.
Stress: Psychological stress can deplete your energy reserves. The mental and emotional burden of adhering to a strict diet, along with external stressors, can increase cortisol levels and disrupt sleep.

Practical Solutions to Combat Diet-Related Sleepiness

If you find yourself feeling excessively sleepy while dieting, here are some practical steps to take:

Eat Nutrient-Dense Foods: Instead of severely restricting calories, focus on consuming high-quality, nutrient-rich foods. This ensures your body has the necessary fuel and vitamins to function properly.
Spread Out Your Meals: Eating smaller, more frequent meals can help stabilize blood sugar levels throughout the day, preventing the energy crashes often associated with going too long between meals.
Prioritize Complex Carbs: Include whole grains, fruits, and vegetables in your diet. These provide a steady release of energy, unlike processed carbs which cause sharp energy spikes and crashes.
Stay Hydrated: Drink plenty of water throughout the day. If you struggle with plain water, try fruit-infused water or herbal tea.
Monitor Your Progress: Keep a food and symptom journal to identify patterns. You may discover that a slight adjustment in your calorie or macronutrient intake makes a significant difference in your energy levels.
Consider Your Exercise Intensity: While exercise is beneficial, listen to your body. If you are consistently exhausted, consider scaling back the intensity or duration of your workouts until your energy levels stabilize.

Conclusion

While it is common and often normal to feel sleepy when dieting, it is not an issue you should simply ignore. Fatigue is a clear signal from your body that something is out of balance. By understanding the root causes—such as an overly aggressive calorie deficit, nutrient deficiencies, or poor lifestyle habits—you can implement effective strategies to restore your energy. Moving towards a more balanced, nutrient-focused diet and prioritizing sleep and hydration will help you combat fatigue and make your weight loss journey more sustainable and enjoyable. If persistent or severe fatigue continues, it is always wise to consult a healthcare professional to rule out any underlying medical conditions.

Authority Link

For more in-depth information on nutrition and energy metabolism, consult the National Institutes of Health (NIH) website: https://www.ncbi.nlm.nih.gov/books/NBK555956/.

Frequently Asked Questions

You feel tired and weak on a diet primarily due to a caloric deficit, where your body is receiving less energy than it expends. Additionally, nutritional deficiencies from cutting out food groups, dehydration, and a slowed metabolism can contribute to these feelings.

To increase your energy, ensure you are eating nutrient-dense foods, balancing your macronutrients, and staying hydrated. Avoid long gaps between meals by having healthy snacks, and ensure you are getting adequate sleep.

Feeling sleepy is common and can be a normal part of the adjustment period when starting a diet. However, if it's excessive or persistent, it's a sign that your body's needs are not being met and you should re-evaluate your diet and lifestyle choices.

Yes, deficiencies in key nutrients like iron, vitamin B12, vitamin D, and protein are known causes of fatigue. Restrictive diets can often lead to these shortages, which can severely impact your energy levels.

Yes, diet can significantly affect sleep quality. Eating a balanced diet with plenty of fiber-rich foods is associated with better sleep, while consuming high-sugar, high-fat processed foods can disrupt sleep patterns.

If your fatigue is a result of dieting, it is usually manageable with dietary adjustments, better hydration, and sleep. However, if the fatigue is severe, accompanied by other worrying symptoms, or continues despite making positive changes, it is important to consult a doctor to rule out underlying medical conditions.

It is not uncommon to experience some initial fatigue as your body adjusts to a new routine and lower calorie intake. This period typically lasts a few days to a couple of weeks. However, if the fatigue is severe or lasts longer, it's important to investigate the potential causes.