Why am I lighter in the morning than at night?

As a very health-conscious person, I weigh myself regularly. I found that if I weigh myself shortly before going to bed, and then again shortly after waking up, there is often a difference of as much as three or four pounds. Do I sweat that much in my sleep? Am I going to the bathroom while asleep and not realizing it? Or, is the mass of the day’s meals somehow just vanishing into the aether (the thermodynamics of which would raise even further questions)?

Well, sweating and metabolism would account for a loss of weight overnight, but not 3 or 4 pounds worth.

Yeah, 3-4 pounds is unusual (I guess if you’re very very overweight, it might be a normal sweat/metabolism weight, but I can’t really speak to that). During wrestling, we’d weigh in in the morning partly because you’re lighter in the morning. Normally I’d drop 1 to 1.5 lbs (when I weighed maybe 170). And this was also being fairly dehydrated, so maybe you’d lose more if you’re hydrated and sweating.

I’ll just note that when you lose weight, most of it is lost via your lungs, not your skin. (Or so goes my recollection of a previous thread.) Think of it in similar terms as that even though a car takes in liquid, it expels gas.

This includes the loss of water (by transpiration rather than sweating).

That will be entirely dependent on temperature. Respiration rate during sleep is very low, and most people keep themselves fairly warm when they sleep, so I would not be surprised to find that for most people that for most people the rate of loss is higher through the skin than through the lungs.

Get a different scale. Maybe in the morning, the scale is colder and stiffer…and doesn’t register all the pounds.

That’s my bet. With a 3-4 pound difference, I’m guessing an inferior scale.

I assume you don’t get up and pee at night (or right after you get up and before you weigh yourself)?

All true - but a pound or so is not at all unusual.

Also, and this is not an answer to your question - you’re obsessed with your health if you weigh yourself every day, or several times a day. Get a hobby. Go help someone. Get out of your own self. Your daily weight is only important regarding your health when you look at it over a period of time - sort of like the difference between weather and climate. Folks who are so self-absorbed that they spend inordinate amounts of time measuring and counting themselves and their foods are missing a whole lot about life.

Your weight is lowest in the morning, after you’ve been to the bathroom but before you’ve had anything to eat or drink. Most of the reason is that you are dehydrated slightly. Also, since you are not eating, your body uses your glycogen stores for fuel. As the day progresses (in addition to the weight of any food you consume), you replenish glycogen stores and gain “water weight.” For this reason, the Tanita scale, which also gives a body fat % reading, is best used late in the day. The scale uses bioelectrical impedance, which is most accurate when tissues are fully hydrated.

http://www.tanita.com/en/science-innovation/

http://primusweb.com/fitnesspartner/library/weight/scale.htm

This was going to be my guess as well. A lot of bathroom scales don’t seem to be very accurate, and I wouldn’t be surprised if yours is sensitive to temperature variations.

I think 3-4 pounds weight loss in the morning is normal. After you eat dinner, your weight will go up by the weight of the food and drink, until it is digested, excess secreted, and metabolized. First thing in the morning, you haven’t eaten for at least six hours. You have lost some water, your food has been digested and metabolized, and you take a healthy bowel movement. After I eat dinner, my weight may go up to 147-149 pounds, depending upon my meal, but in the morning, after a bowel movement, it may be 143 or 144. And I have a very accurate scale.

I’ve also read that alien abductions usually involve the removal of several pounds of various fluids and organ samples. Does your anus tend to feel tender on the mornings when you weighed significantly less?

I think that the 3-4 lbs could be explained with a bowel movement, but I think the OP might have mentioned that. Just the water loss/metabolization alone wouldn’t account for that much weight. As I said, I only have personal experience with the 160-180 weight range and the 1-1.5 lbs that are lost during the night that way, but I would imagine it’s fairly average.

Maybe if the OP could check back in on his/her weight and morning Sir Thomas Crapper habits, we’d be able to find a more definite answer.

I normally urinate before I weigh myself in the morning, so that is probably a contributing factor (though it can’t be that much). Temperature might be a factor as well, since I don’t have air conditioning and my room can get rather warm at night. I’m far from obese, standing at 6’4" and weighing around 183 (+/- the nightly 4, and in pretty good shape). Cheap scale is a possibility, it was only 30 buck and I’ve had it for over a year. Somnambulant trips to the porcelain throne strike me as the most likely solution though, because I do have a history of sleepwalking.

I never really weigh myself, but I can believe a 3-4lb difference overnight. If you have a big meal in the evening then weigh yourself, then urinate (a pint of pure water weighs a pound and a quarter, as we all know*, so a good bladderload could account for the best part of a pound), then spend a night sweating and exhaling moist air plus metabolites (say another pound), then wake up, urinate again and take a dump (say a pound and a half combined) before weighing again.

Just drinking a couple of pints of fluid in the evening will add 2.5lb to your weight temporarily, so it’s not that big of a fluctuation.
*Well, all of us in the UK, anyway. I guess US pints are about a pound.

Not only that, but we’re so ethnocentric that we have a saying…

“A pint is a pound / the world around”