Does Fasting Make You Tired? Understanding Fatigue and Other Side Effects

If you’ve started intermittent fasting and found yourself feeling unusually tired, you’re not alone. Fatigue is one of the most commonly reported side effects of fasting, and many people wonder whether this tiredness is normal or a sign that something is wrong.

The short answer is yes, fasting can make you tired, especially during the initial adaptation period. However, understanding why this happens and how to manage it can help you decide whether intermittent fasting is right for you.

This comprehensive guide explores the connection between fasting and fatigue, along with other potential side effects you should be aware of before starting an intermittent fasting regimen.

Why Does Fasting Make You Feel Tired?

When you fast, your body undergoes several metabolic changes that can lead to feelings of fatigue and low energy. Understanding these mechanisms can help you better manage tiredness during your fasting periods.

Low Blood Sugar Levels

During fasting, your blood glucose levels naturally drop as your body uses up its readily available energy stores. This reduction in blood sugar can cause feelings of weakness, tiredness, and mental fog. Your brain relies heavily on glucose for fuel, so when levels decline, you may experience decreased energy and concentration.

Metabolic Adaptation

When you first begin fasting, your body needs time to adjust to using fat for fuel instead of glucose. During this transition period, which can last several days to a few weeks, you may experience fatigue as your metabolism adapts to the new eating pattern.

Dehydration and Electrolyte Imbalance

Fasting triggers increased water and salt excretion through urine, a process called natural diuresis. If you don’t adequately replace fluids and electrolytes, dehydration can occur, leading to fatigue, weakness, and lightheadedness.

Sleep Disturbances

Some people experience sleep disruptions when practicing intermittent fasting, which can contribute to daytime tiredness. Changes in eating patterns may affect circadian rhythms and hormone production, potentially impacting sleep quality.

Does Fasting Fatigue Go Away?

The good news is that for most people, fasting-related fatigue is temporary. Research indicates that tiredness typically improves as your body adapts to regular fasting periods. This adaptation usually occurs within the first few weeks of starting an intermittent fasting regimen.

Studies have shown that while fatigue is common during the initial days of fasting, many people actually report increased energy levels once their bodies become fat-adapted. Some individuals even experience reduced fatigue compared to their pre-fasting baseline after the adjustment period.

9 Potential Side Effects of Intermittent Fasting

While fatigue is a primary concern, it’s not the only side effect you might experience when practicing intermittent fasting. Here are nine potential side effects to be aware of:

1. Increased Hunger and Cravings

It’s no surprise that hunger tops the list of intermittent fasting side effects. When you restrict calories or go extended periods without eating, your body naturally signals for food through hunger hormones like ghrelin.

Research involving over 100 participants found that those following intermittent energy restriction experienced higher hunger scores than those on continuous calorie restriction diets. However, hunger symptoms typically decrease after the first few days as your body adjusts to the new eating pattern.

2. Headaches and Lightheadedness

Headaches are a common complaint during the initial days of fasting. These “fasting headaches” are usually mild to moderate in intensity and typically occur in the frontal region of the brain.

Several factors may contribute to fasting-related headaches, including low blood sugar levels, caffeine withdrawal, dehydration, and electrolyte imbalances. People who regularly experience headaches are more likely to have them during fasting periods.

3. Digestive Problems

Changes in eating patterns can affect your digestive system, leading to issues such as constipation, diarrhea, nausea, bloating, or indigestion. The reduction in food intake may slow digestive processes, while sudden changes in diet composition can disrupt gut bacteria balance.

Choosing nutrient-dense, fiber-rich foods during eating windows can help minimize digestive discomfort and maintain regular bowel movements.

4. Irritability and Mood Changes

Low blood sugar levels during fasting can affect your mood, leading to irritability, anxiety, and difficulty concentrating. Research has shown that participants were significantly more irritable during 18-hour fasting periods compared to non-fasting periods.

Interestingly, while experiencing increased irritability, many people also report higher levels of achievement, pride, and self-control by the end of their fasting periods.

5. Fatigue and Low Energy

As discussed earlier, fatigue is one of the most reported side effects of intermittent fasting. Low blood sugar, sleep disturbances, and the metabolic transition to fat-burning can all contribute to feelings of tiredness and weakness.

The severity and duration of fatigue vary among individuals. While some people experience significant tiredness initially, others may have minimal symptoms. Most people find that energy levels normalize or even improve after the adaptation period.

6. Bad Breath

Many people notice their breath smells different during fasting. This occurs because when your body burns fat for fuel, it produces ketones, including acetone, which is expelled through your breath.

Additionally, reduced saliva production during fasting can lead to dry mouth, which may contribute to bad breath. Staying hydrated and practicing good oral hygiene can help manage this side effect.

7. Sleep Disturbances

Some individuals report difficulty falling asleep or staying asleep when practicing intermittent fasting. In one study of over 1,400 participants, 15% reported sleep disturbances related to fasting, making it the most frequently mentioned side effect.

However, research findings are mixed. Some studies show no effect of intermittent fasting on sleep quality, while others suggest it may even improve sleep in the long term.

8. Dehydration

During the initial days of fasting, your body releases large amounts of water and salt through urine. If you don’t replace these lost fluids and electrolytes, dehydration can occur.

People new to intermittent fasting may also forget to drink water during fasting periods or simply not drink enough. Dehydration can cause fatigue, headaches, dizziness, and other unpleasant symptoms.

To stay properly hydrated, drink water consistently throughout the day and monitor your urine color. Pale yellow urine indicates good hydration, while dark yellow suggests you need more fluids.

9. Risk of Malnutrition

If not practiced correctly, intermittent fasting can lead to nutrient deficiencies and malnutrition. This is particularly concerning if you do very long fasting periods without adequately replenishing nutrients or if you excessively restrict calories.

Most people can meet their calorie and nutrient needs on various intermittent fasting protocols. However, careful planning is essential to ensure you’re consuming a well-rounded, nutritious diet during eating windows.

Tips to Reduce Fatigue While Fasting

If you’re experiencing tiredness during intermittent fasting, these strategies may help:

Stay Properly Hydrated

Drink plenty of water throughout your fasting period. Aim for at least 8-10 glasses daily, and more if you’re physically active. Consider adding a pinch of sea salt to your water or drinking mineral water to help maintain electrolyte balance.

Start Gradually

Instead of jumping into extended fasting periods, ease into intermittent fasting gradually. Begin with a 12-hour overnight fast and slowly extend the fasting window as your body adapts.

Optimize Your Eating Windows

Focus on nutrient-dense, whole foods during eating periods. Include adequate protein, healthy fats, complex carbohydrates, and plenty of vegetables to provide sustained energy and essential nutrients.

Maintain Electrolyte Balance

Ensure adequate intake of sodium, potassium, and magnesium. These electrolytes are crucial for energy production and can help combat fatigue. Consider including mineral-rich foods like leafy greens, nuts, seeds, and avocados in your diet.

Get Quality Sleep

Prioritize 7-9 hours of quality sleep per night. Maintain a consistent sleep schedule and create a relaxing bedtime routine to support your body’s recovery and energy levels.

Time Your Fasting Appropriately

Experiment with different fasting schedules to find what works best for your lifestyle and energy levels. Some people feel better fasting in the morning, while others prefer evening fasts.

Consider Light Exercise

While you might not feel up to intense workouts initially, light physical activity like walking can actually boost energy levels and help your body adapt to fasting more quickly.

Who Should Avoid Intermittent Fasting?

While intermittent fasting can be safe for many people, it’s not appropriate for everyone. The following groups should avoid intermittent fasting or only try it under medical supervision:

• Pregnant or breastfeeding women
• Children and teenagers
• Older adults with frailty or weakness
• People with current or past eating disorders
• Individuals with diabetes or blood sugar regulation issues
• Those with a history of traumatic brain injury
• People with immunodeficiencies
• Individuals taking certain medications
• People with chronic health conditions

If you have any medical conditions or take medications regularly, consult with your healthcare provider before starting intermittent fasting. They can help determine whether it’s safe for you and provide guidance on how to implement it properly.

When to Stop Fasting

While some side effects are normal during the adaptation period, certain symptoms indicate that intermittent fasting may not be right for you. Stop fasting and consult a healthcare professional if you experience:

• Extreme, persistent fatigue that doesn’t improve
• Severe or prolonged headaches
• Dizziness or fainting
• Extreme hunger or obsessive thoughts about food
• Signs of disordered eating patterns
• Persistent nausea or digestive issues
• Significant mood changes or depression
• Difficulty concentrating or mental fog that doesn’t resolve

Remember that intermittent fasting is just one approach to health and wellness. If it makes you feel miserable or significantly impacts your quality of life, it’s not the right choice for you, and that’s perfectly okay.

Alternative Approaches to Health

If intermittent fasting causes persistent fatigue or other troublesome side effects, many other evidence-based strategies can improve your health:

• Following a balanced, whole-foods diet without time restrictions
• Engaging in regular physical activity
• Getting adequate, quality sleep
• Managing stress through meditation, yoga, or other relaxation techniques
• Maintaining social connections and mental wellness
• Staying properly hydrated
• Limiting processed foods and added sugars

These fundamental health practices are often more important than any specific eating pattern or diet trend.

The Bottom Line

So, does fasting make you tired? Yes, it can, especially during the initial adaptation period. Fatigue is one of the most common side effects of intermittent fasting, caused by factors like low blood sugar, dehydration, sleep disturbances, and metabolic changes.

The good news is that for most people, fasting-related tiredness is temporary and often improves as the body adapts to regular fasting periods. Some individuals even experience increased energy levels once fully adapted.

However, intermittent fasting isn’t for everyone. If you experience persistent fatigue or other concerning symptoms, it may not be the right approach for your body. Always consult with a healthcare professional before starting any new eating regimen, especially if you have underlying health conditions or take medications.

Remember that optimal health comes from a combination of good nutrition, adequate sleep, regular physical activity, stress management, and other lifestyle factors. Intermittent fasting is just one tool among many, and the best approach is always the one that you can maintain sustainably while feeling your best.

Sources:

• National Institutes of Health – PMC Database: Intermittent Fasting Side Effects
• Cell Metabolism Journal: Effects of Intermittent Fasting on Health
• PMC: Intermittent Fasting and Weight Loss
• PMC: Fasting and Cardiovascular Health
• PubMed: Intermittent Energy Restriction Study
• PMC: Fasting-Related Headaches
• PMC: Fatigue and Intermittent Fasting Research
• PMC: Long-term Effects of Fasting on Fatigue
• PMC: Dehydration and Oral Health
• PMC: Alternate-Day Fasting and Sleep Quality