There are a few key reasons why our weight fluctuates over the course of a day, with us typically weighing the least in the morning.
Dehydration Overnight
One of the main reasons we weigh less in the morning is that our bodies become dehydrated overnight. When we sleep, we exhale water vapor through breathing, and we don’t take in any fluids. This causes us to become slightly dehydrated. Since water has weight, losing water through breathing and not replenishing it causes our weight to decrease.
On average, we lose about 1-2 lbs of water weight overnight just through breathing and sweating. This accounts for a good portion of the weight loss we see on the scale in the morning.
Muscle Glycogen Loss
Another factor is that our muscles burn through glycogen stores at night. Glycogen is the stored form of glucose that is used to fuel muscle activity. It is stored in our muscles and liver bound to water molecules.
Overnight, while fasting and at rest, our body still needs energy so it breaks down muscle glycogen stores. As this happens, the water molecules bound to the glycogen get released and excreted from the body.
This loss of muscle glycogen and the associated water release is another reason we lose weight overnight.
Food and Waste Weight
Throughout the day as we eat and drink, we are constantly taking in calories, fluid, and fiber. This adds weight to our bodies in the form of food, water, waste matter, and undigested fiber.
Overnight, as our bodies digest and eliminate, most of this added material gets excreted. We urinate, have bowel movements, exhale carbon dioxide, and lose water through sweat and breathing.
This elimination of waste and undigested material reduces the overall contents of our digestive system, also contributing to lower morning weights.
Changes in Posture
Postural changes between evening and morning can also affect the number we see on the scale.
In the evening after a long day, our spinal discs are compressed from gravity and activity. In the morning after lying down all night, the discs rehydrate and regain lost height. This gain of disc height upon rising makes us slightly taller in the morning.
The lengthening of our spine means the scale doesn’t need to compensate as much for a compressed spine. This can translate into a lower body weight reading.
Loss of Body Fat
While not a primary factor, we do lose a small amount of body fat overnight. When we fast and sleep, our basal metabolic rate remains active and continues to burn calories. A portion of these calories burned comes from stored body fat.
However, the amount of fat loss occurring overnight is minimal. We may lose up to around 0.5 lbs of fat over 8 hours of sleep. This small loss can create a light reduction on the scale.
Changes in Hormones
Fluctuations in hormones can impact the amount of water weight we hold on to. In the morning, our adrenaline levels rise while anti-diuretic hormone levels fall. This promotes urine output and a release of fluid into the bloodstream.
We also have reduced levels of circulating insulin in the morning, which signals our kidneys to excrete more sodium. This sodium loss further reduces water retention.
Morning Variability
In addition to these systemic factors, simple variability can play a role too. Our weight fluctuates within a range day to day based on water intake, digestion, physical activity, etc. Weighing ourselves first thing in the morning reduces variability caused by food and drink consumption throughout the day.
Due to this reduced variability upon waking, the morning weight generally falls at the lower end of our normal range. Therefore, it typically shows up lighter than an evening weigh-in.
Conclusion
In summary, the main reasons we weigh less in the morning include:
- Dehydration from sleep
- Loss of glycogen stores and water in muscles
- Elimination of waste, food, and water weight from digestion
- Height increase from spine decompression while lying down
- A small amount of fat burned overnight
- Hormonal changes promoting urine and sodium loss
These systemic factors combine with normal daily weight fluctuations to produce weigh-ins that tend to be lowest in the morning. Understanding these influences can help explain why the number on the scale changes between morning and night.