1. How Much Body Weight Can Change in One Day
If you weigh yourself at different times, the number on the scale can shift. Health management services explain that weight can fluctuate by 0.5 to 2kg within a single day [1]. Scale manufacturers also note that a change of about 2kg between morning and night is not unusual [3]. To visualize this, 2kg is equal to the weight of 2 liters of water. However, these figures are general guidelines from private companies and vary from person to person. They are not averages from scientific studies.
Is this wave caused by fat? Or is it something else? The difference in weight during the day is mostly due to changes in the amount of water in your body, not an increase in body fat [1]. Eating a meal temporarily increases weight, while sweating decreases it [3].
2. Why the Daily Weight Increase Is Mostly Water, Not Fat
Water is lost even while you sleep. One explanation suggests that about one cup of water is lost through sweat during sleep [1]. Morning bathroom habits also affect the difference between the weight when you wake up and the weight before bed [1].
Nutrition guides state that the total water output for a typical adult is about 2,300mL per day. The breakdown is approximately 1,300mL in urine, 900mL through insensible perspiration, and 100mL in stool [4]. Insensible perspiration is water that evaporates without you noticing. The urine figure is an estimate assuming a middle range of 1,000 to 1,500mL [4].
If you divide the 900mL of insensible perspiration by 24 hours, it is about 38mL per hour, or roughly 2 to 3 tablespoons. Even if you do not feel like you are sweating, water is slowly leaving your body. This is a separate process from active sweating.
3. How Body Temperature Changes From Early Morning to Evening
Your body temperature is not the same all day. According to the thermometer manufacturer’s research institute, temperature is lowest in the early morning, gradually rises to a peak in the evening, and begins to fall at night [2]. The range of change is said to be about 1°C, but this is not a fixed value because it is influenced by the environment [2].
Academic texts provide more detailed measurements. They describe the daily rhythm of core body temperature in healthy people as having an amplitude of about 0.2 to 0.8°C [5]. The lowest point is usually around 5 to 7 AM, about 2 hours before waking up. The highest point is around 8 to 10 PM, about 2 hours before going to sleep [5]. There are differences between men and women and between individuals.
The figures "about 1°C" and "0.2 to 0.8°C" might seem contradictory. Amplitude refers to half the difference between the peak and the trough of a wave. These two sources differ in the body part measured and what they refer to, so they should not be compared directly. Also, the time of the highest temperature varies slightly between sources (evening versus 8-10 PM). We should accept these as rough ranges.
4. How Heat Escapes From Hands and Feet During Sleep to Cool the Body
This temperature wave is connected to sleep. The thermometer research institute explains that when you fall asleep, blood vessels in the backs of your hands and feet open up. This makes the temperature of your hands and feet rise relative to your body [6]. Heat escapes from your hands and feet to the outside, which lowers the temperature inside your body and your brain, helping you fall asleep [6].
If a sleepy child’s hands and feet feel warm, it might be a sign that these heat escape routes have opened. The hands and feet become relatively warmer as the body cools its interior.
5. Why Weight Changes and Temperature Changes Happen for Different Reasons
The weight wave is explained by the input and output of water through eating, elimination, and sweating [3]. The temperature wave is explained by the body’s internal rhythm [5]. Both are normal daily waves where values change from morning to night.
Body temperature peaks about 2 hours before sleep, then drops due to heat release from the hands and feet, allowing sleep to begin [5][6]. At night, your body might be in a slightly strange state: weight is heavier, and temperature is just about to drop.
However, the collected materials do not state whether the weight wave and the temperature wave influence each other. We should understand them as moving for separate reasons.
6. How to Measure Your Own Daily Weight and Temperature Changes
If you have a thermometer and a scale at home, you can observe your own daily waves. With an adult’s help, try keeping a record. Weigh yourself just after waking up and just before bed, wearing the same clothes and in the same place. Compare the difference between morning and night over 3 days [1].
Measure temperature at the same spot (such as under the arm) just after waking up and just before bed. Values change if the spot or time is different, so keep conditions consistent when comparing [2].
This record is for seeing the shape of the wave, not for losing weight. If you are worried about your health, do not rely on the record; consult an adult or a medical professional instead. For more reading, the thermometer institute’s article on "Sleep and Body Temperature" summarizes the relationship simply [6].