Once a year, I am fasting for 24 hours. No food or drink at all for whole 24 hours.

Recently I thought about what is happening to me during those 24 hours, and realized there might be something strange going on. I will try to describe the things I find strange:

  1. I do not feel thirst or hunger at all. Not even after 20 hours.
  2. I still urinate after 10, 15, and 20 hours, and unlike what I would expect from not drinking, the color is not very dark. (actually, same as when I drink normally)
  3. I can still perform energy consuming tasks after 20 hours without eating or drinking anything. I'm not talking about running in the park or lifting weights, but I can walk, do house chores etc, and only after 20 hours or so the body starts to "protest" in the form of headache.

All of the above led me to think that maybe when knowing it's going to be deprived of energy, the body enters "Power Save" mode, allowing it to function as usual under those conditions? (to a limit of course, but still - I would expect hunger, thirst, and headaches to appear few hours into fasting)

  • $\begingroup$ I just voted to close, because I find it an anecdotal question. Maybe the headache is caused by a lack of coffee etc etc. It sounds all like a personal monologue on your experience. $\endgroup$ – AliceD Aug 23 '15 at 15:32
  • $\begingroup$ @AliceD fair enough, wasn't sure if it fits here or not. Still giving it a chance to see if others find it off topic as well and in this case will delete. (for the record, the headache is really not the issue here) $\endgroup$ – Shadow9 Aug 23 '15 at 15:34
  • $\begingroup$ Please leave the question as is, it is just a single vote - no harm done :) And I understand the headache is just a side note. I just took it as an example. If you could try to extend the horizon of your question to broaden its scope I'm happy to retract the vote. Btw: fasting = good, no liquids = nono afaik $\endgroup$ – AliceD Aug 23 '15 at 15:37
  • $\begingroup$ Thanks @Alice, not going to delete yet. Any tips how to broaden the scope? $\endgroup$ – Shadow9 Aug 23 '15 at 16:54

If you are performing the fast for ritualistic or spiritual purposes, then that mental state likely overcomes the feeling of thirst or hunger, especially if you are an otherwise healthy person.

As for the physiological aspects. The Human Body is made up of 2/3 water, and as your kidneys are still functional, they are constantly filtering, and creating urine. The body can last a few days without water.

As for food, the body has about a one day supply of sugar, assuming limited exertion, that it can consume before it begins to break down other substances. Then there are the fat reserves. The average person will likely have on the order of 7 to 15 kilograms of fat in reserve. There are 9 calories per gram of fat, so roughly 63 to 135 thousand calories worth of energy. Since the average adult will use about 2000 calories a day, then you can see that burning body fat alone would allow about a months worth of energy.

I am not saying that these are optimal conditions, but the body of a healthy human adult can easily withstand a 24 hour fast with no adverse effects. You would likely encounter more problems if you tried to go without sleep for the same period of time than if you refrained from food or water.

  • $\begingroup$ So in essence, in reply to the actual question of "Can the body enter power save mode" you say "No, it's just a mental state"? $\endgroup$ – Shadow9 Aug 23 '15 at 16:53
  • $\begingroup$ I am saying that while feelings of thirst and hunger are sensations derived from physiological effects, they are experiences which are interpreted by the brain, and as such humans have developed the cognitive capacity to act against their instincts, and things such as meditation, prayer, repetitive ritualized behaviors can overcome the perceived sensations of the body. I said that if the reason for your fast was related to such a behavior it is likely that not experiencing the sensations of hunger and thirst has far more to do with the focus of your state of mind than the signals of your body $\endgroup$ – AMR Aug 23 '15 at 17:23
  • $\begingroup$ Some mammals hibernate, and in those cases, actual metabolic activity is drastically reduced. Humans didn't evolve to do this. The closest we come to this are situations of hypothermia. As far as power save... It is likely that if you have enough of a calorie deficit for a long enough period of time, you could end up needing more rest and not being able to exert yourself, also in the absence of food our body's metabolic processes are altered, but a single day isn't going to shut you down or alter your metabolism in any significant way, assuming again you are a health, non-pregnant, adult. $\endgroup$ – AMR Aug 23 '15 at 17:28

The human body can last several days without food and (to a lesser degree) water, and often had to. I myself and certainly many others have not had to experience that tho, as modern Western society has pretty good countermeasures against this.

After 24 hours, your body has still sufficient amounts of water, unless it was dehydrated earlier already. Contrary to what an American stereotype would suggest, a few hours without food won't do any harm.

When looking at longer times, however (which is closer to the core of your question!), this is not as easy anymore. Sadly I'm not familiar enough with this topic to give sensible intervals, but there are certainly many papers about this that could explain this in detail, if you're interested.


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