Why You Weigh Less in the Morning

In my last post, I posed the question “Why is my morning weight lower than my bedtime weight?” It’s true, you know. Your lowest weight is always in the morning.

Let’s see if I can explain it.

Basically, overnight weight loss is a combo of (1) water lost via the skin and respiration, (2) basal metabolic caloric expenditure and (3) urine loss in the morning. Water loss, excluding urine and feces, accounts for over 80% of the weight lost while sleeping.

Overnight weight loss may vary depending on how much time one spends in Slow Wave Sleep (SWS) vs REM sleep, with longer duration of SWS correlating with higher sleep weight loss. Slow wave sleep has been called the restorative phase of sleep, and increased exercise during the day leads to an increase in time spent in SWS during the night.

Energy expenditure during sleep varies with varies with circadian rhythms as well as sleep cycle. So one could hypothesize that disruption of these rhythms night lead to aberrations in weight.

The sleeping metabolic rate in non-obese individuals appears to be higher than that of their obese counterparts. Meaning that thin folks may stay that way, not just because of what they do while they are awake (namely, eat less and exercise more), but also because they burn more calories while they are sleeping. (I know. Totally unfair.)

Sleep deprivation has been found to be associated with obesity in adults, and appears to be a possible cause for the rise in childhood obesity. In this regard, I will tell you that I used to stay up till the wee hours blogging. But I don’t do that much anymore, and am definitely getting more sleep. I wonder how much that has contributed to my weight loss success?

Bottom Line

While most of your overnight weight loss is water loss, don’t discount the loss that comes from the energy you burn while you are sleeping. It may not be a lot on a given night, but multiply it by 365 nights a year and it adds up.

The amount of energy you burn while sleeping seems to depend on how well and how long you sleep, so make sure you get enough shut-eye.

And while you’re awake, get some exercise. Exercise improves the quality of your sleep, and that may lead to more weight loss.

Now if you’ll excuse me, it’s way past my bedtime.

13 Responses to Why You Weigh Less in the Morning

  1. Water lost by the body has two components: ingested water and respiratory created water. Loss of ingested water leads to weight loss; but loss of respiratory water should not. Water(H2O)) has a molecular weight of 18; the oxygen atom acocunts for 16/18 of the entire weight of water and the oxygen atom come from air. 16/18 of the weight of this water comes from the outside never contrubutes to body weight,

  2. Ah…chemistry. I love chemistry.

    While the anonymous poster is correct as it related to the atomic weights of both H and O, what he is not considering is that with respiration two (2) oxygen atoms come off in the exhale!

    OK, so in a nutshell, a study would need to be done where volume of air “in” is measured as well as volume of air “out”. In addition, relative humidity of both the air in and the CO2 out should be measured to determine if in fact the equation becomes balanced. There have to be additional sources of O contributing to the double O exhale. I suspect that in the lungs, some water is lost from the body to get to CO2, so that would explain why there is water loss due to respiration during the night.

    My guess is that we breathe deeper and more efficiently when sleeping then when we are just sitting at our respective desks writing comments on a blog (OK, a really good blog…).

  3. Congrats on the weight loss. I’ve decided it’s time for me to try again. 😛

    I always assumed that you weigh less in the morning because you get up and urinate and then weigh yourself, so there’s been 8 hours where you’ve been making urine and breathing and all of that stuff but not eating or drinking.

  4. On a similar note, can you explain why on earth I would GAIN 2 poundas AFTER a bowel movement? I always weigh before and after (I’m twisted that way) and I always weigh 1-2 pounds MORE afterwards. Maybe I’d lose lots of weight if I never went #2 again.

  5. I have never really thought much about why I weigh less in the morning but I often think about the damage I am doing to my body by being a shift worker and mother of four. Switching back and forth between days and nights at least once a week has messed up my own circadian rhythm. AND made it just that much harder to take off the pounds I put on during my fourth pregnancy. The upside to my shift work is that it is VERY active…I am a Respiratory Therapist in a medium sized hospital and spend a lot of time on the stairs, throw in some good ole fashioned CPR and my cardio for the day is usually covered.
    Congratulations on sticking with your weight loss plan and the amazing results! You go girl!

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  7. I decided that it time I got serious about weight loss. Your blog has finally motivated me to DO SOMETHING.
    1) I used one of my coveted BB&B 20% coupons and bought a bathroom scale.
    2) I made an appt to see my PCP (It’s been so long, what’s his name again?)
    3) I had to revisit your posting on weighing less in the morning because now that I have a new toy, I weigh myself all day long. It’s the last thing I do at night and the first thing I do in the morning (before going to the bathroom)

    I still don’t get it.

    Last night I lost 2 pounds. I did spend about an hour tossing and turning because I thought I heard a mouse in the apt.
    I have been reading Molecular Biology & Genetics grant proposals for the DOD and my head is swimming with in-vivo studies, mice with breast cancer….

    • Off topic, but I am curious – what are the genetics grants for? My daughter has a really rare (as in she is the only one) genetic disorder and we are military. Just wondering what the DOD is doing in that area.

  8. I weigh 5-6 pounds less in the morning than at night, I suppose that means I must have a high metabolic rate, just seems like that is an aweful lot of weight in 6-8 hours…

  9. I just want a simple answer to why I weighed 5 pounds less than I did the night before. I am watching my weight drinking at least 8-10 glasses of water and taking the vegatarian approach. I always assuned the scale doesn’t lie and if intial you lose water weight the first 5-10 pounds. So I just want to know when to weigh myself to get accuracy.

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