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Sending a kid to bed without supper was and is a punishment technique. Whether or not it is good parenting, it is easy to understand why this is a punishment for most kids (and adults). Trying to get to sleep on an empty stomach is more difficult and the lying awake waiting for the release of sleep can be uncomfortable.
Fasting induces nocturnal hypothermia in animals and food deprivation in general tends to increase vigilance. This makes sense evolutionarily. With an empty belly the animal needs to be alert for feeding opportunities and go into an energy conservation mode (hypothermia to some extent).
Unfortunately there is little scientific examination on the effect of fasting on sleep. A small study found that over a short period of limited fasting sleep quality improved by measured number of nighttime awakenings and periodic leg movements.
Lifehacker reports a method of “re-setting” your sleep cycles with a 16-hour fast. Planning your wake-up time and not eating 16 hours before that wake-up is the key. The system relies on the fact that the body’s internal cycles are not independent and hunger and sleep propensity drives can be intertwined. Light affects the circadian cycle but so does the consumption of food.
This makes evolutionary sense. If a person in a food-scarce world (as our ancestors lived in) can’t find food, he might as well sleep to conserve energy, get the sleep needs out of the way, and wait until the prospects for food to improve.
Muslims who fast during daytime for the month of Ramadan may be able to help handle their regimen if they are able to sleep during the day. The body’s built-in appetite decline during the main sleep period helps.
Ramadan fasting is a pattern called “intermittent fasting” in scientific studies. One problem with finding out what happens during Ramadan is the dearth of published scientific investigation. Studies of food deprivation almost always look at longer periods of deprivation than just the period of daytime. Indeed, the normal human pattern includes a fast of 10 or so hours overnight, ending with a breakfast (which derives from “break fast”). The difference is that sleep is tied with hunger patterns and in normal functioning we have no hunger when we sleep at night. The Ramadan pattern is to fast during hours when we normally are awake. Those observing the Ramadan fast often have to squeeze food intake between sundown and bedtime. Some attempt to switch their sleep time to help cope with hunger – essentially shifting part of the sleep period to daylight hours.
The studies of Ramadan fasters find a common pattern in many people is a reduction in sleep latency time (people got to sleep faster) and time spent in Stage R was less during the third week of Ramadan fasting This might sound counterintuitive since the subject eats after dark but before going to sleep.
There is no general effect on the total sleep time. Studies have suggested sleep time increases and other studies suggest sleep time decreases. Where in the year Ramadan happens may have an impact. The lunar month moves through the calendar year so sometimes it is in the summer during long days and sometimes it is in the night with short days. Ramadan fasting does not appear to affect daytime sleepiness. The struggle people feel during the day is more with hunger than sleepiness (compared to the rest of the year).