The science behind overnight weight gain

Seeing the numbers on the scale go up overnight when you’re on track with your health routine can be confusing—but this probably isn’t an actual gain in weight. Learn all about the science behind weight fluctuations and what you can do to deal with them.
Monica Karpinski
min read
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Quick summary

  • Overnight changes in weight are more likely to be a fluctuation rather than a true gain 
  • Sometimes, there can be changes in the body that temporarily make us heavier, such as bloating 
  • Water retention is a common cause of bloating. This can happen when we don’t drink enough water or eat too much salt 
  • Hormonal shifts during the menstrual cycle have also been linked to bloating 
  • Weight fluctuations are normal and not a reflection of the progress you’ve made on your health journey

You’re a few weeks into your new diet and exercise routine and are feeling pretty good: lighter, more energised, and healthier. Then you stepped onto the scale this morning and the number was higher than it was yesterday. How did this happen?

Chances are that this is a fluctuation in weight rather than an actual gain. In theory, to put on a kilo in a day you’d need to eat at least 4,400 more calories—about four medium Big Mac meals—more than your body uses to function[1], which is around 2,200[2]. 

But even if you did do this, overeating once isn’t likely to have much of an effect—our metabolism can generally compensate by working harder than usual to keep blood sugar and fat levels in check.[3] 

So if an excess of calories doesn’t explain this sudden increase in weight, what can?

One in three people will feel bloated, and therefore heavier, at some point in their lives

Lacy et al., Gastroenterology & Hepatology

Weight fluctuations are perfectly normal[4] and can happen for a range of reasons, including bloating due to water retention or shifts in hormone levels. Here, we’ve broken down some common causes of overnight changes in weight to help you figure out what’s going on. 

What causes overnight weight gain?

True weight gain happens when we consume more calories than our body uses, and then this surplus of energy is stored as fat.[5] We might temporarily gain weight, though, if there’s a change in the body that fleetingly makes us heavier—for example, being bloated. 

Around one in three people will feel bloated at some point in their lives.[6] Bloating can happen from overeating or water retention, or because of an underlying condition like polycystic ovary syndrome (PCOS)[7] or irritable bowel syndrome (IBS).[8] 

Folks with type 2 diabetes can also experience bloating after eating due to a complication called gastroparesis, which is when the stomach takes longer than normal to empty.[9] About 10% of people with type 2 have it.[10] 

We’re going to look at some of the more common reasons for temporary weight gain, but if you think something else might be going on with your body, be sure to get checked out by your doctor. 

You didn’t drink enough water

We all know how important water is for our health—it transports nutrients to our cells and organs, controls our body temperature, and generally keeps us alive.[11]

The amount of water our body uses compared to how much of it we take in via food and drink is called our water balance.[11] This balance is tightly regulated by our body’s systems, and if something happens to upset it, the body will try its best to compensate and bring levels back to normal. 

When we don’t drink enough water, our kidneys make up for this by reabsorbing salt and decreasing our urine output in order to retain the water that we do have.[11] 

This can cause bloating, and therefore, an increase in body weight.[12]  

However, not having enough water to the point where we’re dehydrated can make us lose weight, because the total amount of water within the body is less. 

This isn’t a good thing—dehydration can cause problems with thinking and memory, muscle strength, and more. [13] Having a dry mouth or tongue, a headache, or feeling tired and sluggish can be warning signs that you’re in need of some water.

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You ate too much salt 

How much water our body retains or releases is closely linked to how much salt we consume.[14] 

Levels of salt and water in our blood exist in balance with each other and it’s our kidneys’ job to keep it that way. This balance is essential in maintaining key bodily functions like nerve activity and muscle function.[15] 

When we eat too much salt, the kidneys try to restore the balance by retaining more water.[16] And, as we know, water retention can cause bloating.

This is the point where we’d probably start feeling thirsty, which is our body giving us the signal to increase our fluid intake so that it can release the excess salt through urine.[14] However, some newer studies have pointed out that more research is needed to fully understand the mechanisms at play here.[14]  

Your period is due 

If you’re a person who menstruates, have you ever felt bloaty before your period? Well, you’re in good company—according to one study, 77% of people had bloating in the week leading up to their period while 63% had it throughout.[17]  

The jury is still out on why exactly this happens. A link between the hormones oestrogen and progesterone, which fluctuate throughout the menstrual cycle, and fluid retention has been noted in scientific research[18] but we need more evidence here to better understand this relationship.[19]  

77% of people feel bloated in the week leading up to their period

Bernstein et al., BMC Womens Health

What we do know, though, is that oestrogen and progesterone can influence the systems that manage salt and water levels. Both hormones can interact with tissues involved in fluid regulation, including within the kidney.[18]

Dealing with overnight weight gain 

Seeing the numbers on the scale go up when you’re trying to lose weight healthily can be tough, but in most cases, weight fluctuations are nothing to worry about. 

It might feel counterintuitive, but one way to deal with water retention bloating is by drinking more water.[11] When we give our body an adequate amount of water it doesn’t need to retain its own stores. Plus, having enough water means that excess salt can be flushed out as urine.[20] 

Eating less salt can help here, too—if the body no longer needs to balance out high levels of salt, the extra water it’s holding can be released.[20]  

Broadly, it’s recommended that men drink about 4 litres of water per day while women should have just under 3, but these vary depending on the climate where you live and how physically active you are.[13]

And then, there’s the mental load of dealing with changes to your body. Because of the social stigma around larger bodies, people dealing with weight management can have trouble seeing themselves in a positive light. Some have said that they feel a persistent sense of defeat.[21]

But with the right support, there’s no reason why you can’t live as your best self while making changes to support healthy weight loss.

For example, one study found that talking therapy (CBT) was helpful in developing a healthier relationship with food intake and body weight,[22] while another has shown that getting positive reinforcement from our social support networks helps, too.[23] 

Any health journey has its ups and downs, and weight management for people living with type 2 diabetes is no exception. What’s for sure, though, is that a temporary blip on the scales doesn’t define your journey and is no reflection of the progress you’ve made.


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