“Do you have any suggestions to improve sleep when fasting?”

Ask Dr. Jason Fung

There are tons of questions about intermittent fasting, like these:

  • Do you have any suggestions to improve sleep when fasting?
  • Should you fast if your cortisol level is high?
  • Would an 800 calorie low-carb intake for 8 weeks be classified as a fast or calorie restriction?

Dr. Jason Fung is one of the world’s leading experts on fasting for weight loss or diabetes reversal. Here are a his answers to those questions and more:

Do you have any suggestions to improve sleep when fasting?

Hi Dr. Fung,

I’ve had pretty good success with weight loss and general health improvement using alternate day fasting but one issue still bothers me. On fasting days I have trouble getting to sleep and getting enough sleep. I partially mitigate this with a flexible work environment that’s let me start work when I want but I would like to get to a place of better sleep hygiene. I’ve tried meditation and melatonin and both these help but haven’t fully addressed the side effect.

What strategies have you seen people use to deal with this that worked?


Dr. Jason Fung: This is a common problem. The increase in nor-adrenalin and cortisol can disturb sleep. Often, we advise people to only go to sleep when tired. This can sometimes mean cutting sleep time down to 3 hours! Most who do that find they are still fully alert, so it’s fine.

Otherwise, we suggest proper sleep hygiene and perhaps melatonin supplements.

Should you fast if your cortisol level is high?

My cortisol level is 913, but it should be between 150-660. I’ve read that hunger increases cortisol. Are there optimal ways of fasting? Is fasting longer than 24 hours acceptable?

My insulin levels are also high, and I’m working on it with LCHF.


Dr. Jason Fung: Yes, fasting may increase cortisol, which is one of the counter regulatory hormones that is meant to increase glucose in the blood during fasting. Whether it is safe depends upon your medical history, and you would need to discuss with your doctor.

Would an 800-calorie low-carb intake for 8 weeks be classified as a fast or calorie restriction?

In the new publication by Dr Michael Mosley “The 8-Week Blood Sugar Diet”, 800 calories on a low-carb diet is suggested (followed by a Mediterranean low-carb diet, using 5:2 intermittent fasting if necessary). Would this be considered to be a form of fasting or simple calorie restriction?

Would a better use of calories be, for example, 1100 calories alternated with 500 hundred calories? This would match the 800 calories daily.

As a 64 year old woman 5’1″ (155 cm) and weighing 179 lbs (81 kg), I am interested in trying Dr Moseley’s suggestion (or the alternate day version of it) as I lose weight very slowly using just LCHF and 16:8 fasting. I am not diabetic, but am presumably insulin resistant. When I attempted to do a water fast for several days you suggested that I stop as I started to feel very unwell.

With thanks,

Dr. Jason Fung: The 800-calories a day diet for 8 weeks is not sustainable for longer periods of time. It is a good temporary fix, but not one I would recommend. I use intermittent fasting instead – some days will have zero calories and other days will be 2000.

Long term caloric reduction is doomed to fail, as the body adjusts to the lowered caloric intake. Fasting, on the other hand, has been used for thousands of years successfully.



Intermittent Fasting for Beginners

More questions and answers

Earlier Q&A sessions with Dr. Fung:

Many more questions and answers:

Intermittent Fasting Q&A


Read Dr. Jason Fung’s new great book The Obesity Code for many more insights:

The Obesity Code

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