I won't dive too deeply into the benefits of fasting for more than 24 hours here. Rather, I will lay out the process I recommend for undertaking these kinds of fasts.
For a complete guide to fasting, check out my book "The Fasted Lifestyle", which is linked at the bottom of this page. In the book, I reference over 40 scientific journals and explore what the research has to say. Here, I will discuss my personal opinion, which also shouldn't be taken as medical advice.
A 24-hour fast can be practiced as frequently as weekly, whilst extended periods of fasting beyond this should be practiced less regularly. For example, 48-72 hour fasts can be practiced safely on a quarterly basis, so four times a year.
My recommendation for a 24-hour fast is to have a lower carbohydrate meal (so you can reach a fat adapted state quicker during your fast) in the early afternoon of day 1, for example, at 3/4pm. This brings the end of your fast to 3/4pm the following day with a sufficient period for eating on both days. This also makes sleeping much easier on the night of day 1, and breaks down the fasted period with the time that you sleep.
The game changes slightly when you extend your fast such that it requires a full day, from waking up to going to sleep, without eating. Especially for the first time. Once again, begin your fast with minimal carbohydrates in your system to speed up to journey to a fat-adapted state - this final meal can be later in the day prior to the day long fast.
In this case, I would recommend doing something active such as a walk or steady cycle (on a static bike, not on the road) in the period where you expect to find difficulty abstaining from food. For me this is 5-7pm. This is because being active suppresses hunger, and will also pass the time and carry you through a difficult period to fast.
It will also force your body to use any remaining carbohydrates in your system, causing you to reach a fat-adapted state quicker. Once you reach a fat-adapted state you will start to feel better, because your body is now using your fat storage for energy.
Unfortunately, every angle cannot be explored in one idea, but some over-riding suggestions I can give you are:
- Drink water, but don't over-drink because it will flush out the sodium and electrolytes from your body. Do not stress over this, simply drink when you are thirsty.
- Stay relaxed, fasting is a mental challenge, and keeping calm is essential.
- Do not overexert yourself, both mentally and physically, especially if you are new to fasting.
- Avoid caffeine if you are fasting for gut health - if you are purely water fasting, drink water and boiling water like it is tea (surprisingly this satisfies the "want for consumption"). Caffeine sources can also dehydrate you as natural diuretics.
- Most importantly... work your way up to longer fasts. Start by practicing intermittent fasting so you can become accustomed to using fat for energy (metabolically flexible).
Again, you can read all about fasting inside my book, which covers all the angles!