How Much Weight Do You Lose Overnight?

If you’ve ever started a diet or seen a dietician, you were likely advised to weigh yourself at the same time every day. This is because your weight can fluctuate quite dramatically during a 24-hour period.

If you weigh yourself first thing in the morning, you’ll probably weigh less than you do if you step on the scale just before you go to sleep. What you’ve eaten and drunk during the day influences your weight, but your weight also changes overnight while you sleep.

But why, and how, does this happen?

Water weight loss

Approximately 60% of your body weight is made up of water.¹ To keep well hydrated, depending on your body size and level of activity, you will need to drink anywhere between two and three liters of water per day. This water intake will be reflected on the scales.

The two main ways in which your body loses water are through sweating and urinating. During the day, you replace the water that you lose by drinking more. Overnight, you are less likely to replace the water you lose through sweating and urinating.

Sweating

On average, you sweat about 25ml per hour of sleep under temperate conditions (around 85 degrees Fahrenheit).² If you sleep for an average of eight hours, that’s around 200ml of sweat per night. This would equate to a drop in weight of approximately 200g overnight. If it’s a very hot night, you may well sweat more, leading to further weight loss.

Furthermore, vigorous exercise during the day also raises your core temperature. This can persist after you stop exercising, leading to increased sweating during the night. Even eating very spicy foods before bed can lead to increased sweating while you sleep.

There are certain physiological states, such as menopause and fever, during which you can also sweat excessively at night. Heavy sweating that results in you changing your sheets will lead to an even lower figure on the scales in the morning.

Unfortunately, any weight that you lose due to sweating overnight is only short-term. As soon as you start drinking and rehydrating, that weight will return.

Urination

Normal nocturnal urine production should be around 0.5 to 1ml/kg/hour.³ So, if you weigh 70kg and sleep for 8 hours, you should produce between 280ml and 560ml of urine per night. If you aren’t drinking anything during the night and empty your bladder before weighing yourself, your weight could easily be up to half a kilogram lighter than it was before you went to sleep.

As with the case of excessive sweating at night, this is weight that will return as soon as you rehydrate.

Carbon dioxide weight loss

Your metabolism refers to the processes continually going on inside your body that allow you to function. This includes breaking down food in order to build and repair the body. Your body will burn a certain amount of energy (calories) throughout these processes to maintain itself.

Fats and carbohydrates can be thought of as fuel to keep your body able to do all the things it needs to on an ongoing daily basis, such as breathing, keeping your heart beating, and digesting your food. Did you know that a by-product of the metabolic process is carbon? This is excreted as carbon dioxide, exhaled when you breathe out.

Resting metabolic rate drops during sleep by about 15%.⁴ Nevertheless, despite this drop, you still excrete relatively significant amounts of carbon dioxide during sleep. One estimate has suggested that the average human excretes 0.9kg of carbon dioxide daily.⁵ Obviously, this will be less if we only take nighttime carbon dioxide loss into account, but it will still contribute to making you lighter when you step on the scale in the morning.

Can you gain weight overnight?

One of the most common causes of overnight weight gain is a high salt intake.

If you eat a meal that is very high in salt before going to bed, you may notice that you actually weigh more in the morning than you usually do. This is because salt causes fluid retention.

Instead of excreting water as urine, your body holds onto the water to “dilute” the increased salt in your system. It’s possible to retain up to 1.5kg⁶ of fluid after a very salty meal. It may take some time for your electrolytes to rebalance and your body to excrete the excess salt and fluid.

Is there a longer-term relationship between sleep and weight?

The weight you lose each night may be short-term weight loss, but there is a deeper, more long-term relationship between sleep and weight loss.

Studies have shown that poor quality sleep and disrupted sleep can cause weight gain over the long term.⁷ This is due to sleep deprivation interfering with our hunger regulating hormones, increasing appetite, and the likelihood of poorer food choices and overconsumption.  Ensuring that you have a good night’s sleep every night will help prevent weight gain.

The lowdown

It is normal to lose weight overnight, meaning that your weight in the morning will often be lower than your weight in the evening.

The amount of weight you lose overnight will depend on how much you sweat at night and how dehydrated you are in the morning. You may sometimes gain weight overnight if you have fluid retention due to eating a very salty meal just before bedtime.

There is another relationship between sleep and weight, however, and ensuring that you have a good night’s sleep every night will help to prevent weight gain in the long term.