Why do I weigh less in the morning?

We get lighter over night, but unfortunately not enough to start a new sleep-based weightloss plan.

26th January 2018
Why do I weigh less in the morning? © Getty

Asked by: Finbar McDonnell, Berkhamsted

Because you are alive! The chemical reactions that sustain you all require energy, and even though you aren’t eating in your sleep, these metabolic processes are still converting glucose molecules into carbon dioxide (CO2) and water. The air you breathe out contains 4 per cent CO2 and is saturated with water vapour. In a typical eight-hour sleep you will exhale 2,100 litres of air, containing 27g of water and 84 litres of CO2. The carbon in the CO2 weighs 42g.

While 69g in a night doesn’t sound like much, you also lose weight from your sweat, the saliva you dribble onto the pillow, and from the skin flakes you shed into the sheets. This is why you need to change the bedding every now and again.

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