Relative humidity is a measure of how much moisture is in the air compared to the maximum amount of moisture the air can hold at a given temperature. At night, temperatures typically drop. As the air cools, its capacity to hold water vapor decreases. This results in an increase in relative humidity because the amount of moisture in the air remains relatively constant, but the cooler temperatures cause the air to become saturated more easily.
During the day, temperatures rise due to sunlight, which warms the air and increases its capacity to hold moisture. Even if the amount of moisture in the air remains the same, the higher temperatures lead to a decrease in relative humidity. Thus, you can observe that as the sun heats the atmosphere throughout the day, relative humidity tends to drop.
In summary, the daily cycle of temperature fluctuations is a key factor in the increase of relative humidity at night and its decrease during the day.