There are a few possible explanations for why it might feel hotter in your room at night when it gets colder outside, even if you're not using any heating sources:
Insulation: During the day, sunlight may warm up your room, and the walls, furniture, and other objects inside absorb some of that heat. When the outside temperature drops at night, the heat stored in these objects can slowly radiate back into the room, causing it to feel warmer.
Poor ventilation: If your room lacks proper ventilation, the warm air inside can get trapped. As the temperature drops outside, there may be less natural air movement to exchange the warm indoor air with cooler outdoor air. This stagnant warm air can make your room feel hotter than the outside temperature suggests.
Human activity: Our bodies generate heat, and if you're active or have several people in the room during the evening, the combined heat output can raise the temperature in the space. This effect may become more noticeable at night when the ambient temperature drops.
Humidity: The level of humidity in the air can affect how we perceive temperature. Higher humidity can make the air feel warmer, while lower humidity can make it feel cooler. If the outside air becomes drier at night, the relative humidity inside your room might increase, leading to a sensation of higher heat.
Perception and thermal regulation: Our perception of temperature can vary based on our activity levels, clothing, and individual sensitivity. At night, when we are often more relaxed and resting, our bodies may be less active in cooling down through perspiration. This reduced cooling effect can make us feel warmer in the same environment compared to when we're active during the day.
It's important to note that these factors can vary depending on the specific conditions of your room, such as insulation quality, airflow, humidity levels, and personal preferences.