Are trouble sleeping and anger common while fasting?

Ask Dr. Jason Fung

Have extremely obese patients been able to reach their ideal weight goals using fasting? Does drinking MCT oil speed up weight loss during fasting? And are trouble sleeping and anger common while fasting?

It’s time for this week’s Q&A about intermittent fasting and low carb with Dr. Jason Fung:

Trouble sleeping and anger while fasting?

Hello Dr. Fung, I have tried fasting a number of times in the past and found that I cannot sleep very well and feel aggitated/angry during the night/day. It does not matter if it’s a 24-hour fast or 16:8 type. I usually wake up with a thumping heart around 2 AM and cannot get back to sleep. Am I not suited for fasting or am I doing something wrong?


This is common and likely due to the increased noradrenalin. During fasting, insulin drops and the counter-regulatory hormones surge (including nor-adrenalin, growth hormone and cortisol). There is general activation of the sympathetic nervous system (the so-called flight or fight response). This sometimes results in excessive energy and difficulty sleeping. People may also feel irritated or easy to anger.

Most often, we advise people to change their sleeping pattern (go to sleep later or wake earlier) to compensate.

Dr. Jason Fung

Have extremely obese patients been able to reach ideal weight goals?

I started at 732 lbs (332 kg) about 8 months ago and am down to 603 lbs (274 kg) as of today. I’m a long way off my goal and want to know if people have been able to lose 300 or more pounds (136 kg) using a combination of fasting and LCHF? And will I find it harder to lose weight as I get closer to my goal?

Thanks for your time!!!

Yes, some people have lost that amount of weight, but it often takes time. Generally the first pounds are the easiest. There are no rules, but much of the initial weight loss is water and this will slow down.

Dr. Jason Fung

Does drinking MCT oil expedite weight loss during fasting?

Do you recommend MCT oil over say, butter or coconut oil, in my morning coffee during IF (16:8)? Will it expedite weight loss by (supposedly) increasing my ketone levels?


I don’t have any strong data to say that one is better than another. The point of adding pure fats to coffee is to provide calories while keeping insulin low. It may help you feel more full, but does not increase weight loss directly. It may help weight loss efforts by making fasting easier. This is a variant called ‘fat fasting’ and works well for many. Others may find it completely ineffective. I recommend trying it and seeing how your own body responds.

Dr. Jason Fung



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Dr. Fung has his own blog at He is also active on Twitter.

His book The Obesity Code is available on Amazon.

The Obesity Code

His new book, The Complete Guide to Fasting is also available on Amazon.


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