From Google AI:

Here are the primary reasons why your freezer temperature might only rise noticeably at night:

1. ❄️ The Automatic Defrost Cycle

All modern frost-free freezers and refrigerators must periodically warm up to melt any frost buildup on the evaporator coils.1 This is the defrost cycle.

• Timed Cycles: The defrost cycle is typically controlled by a timer or a circuit board and is often programmed to run at regular, fixed intervals, such as every 8, 10, 12, or 24 hours.2

• Night-Time Scheduling: Manufacturers often schedule the defrost cycle to run when the refrigerator is least likely to be opened, which is usually overnight. This minimizes the impact of the temporary temperature increase on your food, as the doors aren't being opened to let in more warm air while the freezer is already warming up.

• Observed Rise: During the defrost cycle, a heating element warms the coils to melt the ice, causing the internal air temperature—especially near the coils—to rise for a short period (typically 20-45 minutes). If your freezer is not very full (lacking thermal mass from food), the air temperature will spike more dramatically.3

2. 🌡️ External Factors Amplified Overnight

While the defrost cycle is the most likely cause, other factors could contribute to a nighttime-only rise:

• Less Thermal Mass (Empty Freezer): If you use less of the freezer space, there is less cold food to absorb the heat during the defrost cycle, leading to a higher, faster spike in the air temperature that your thermometer is registering. This effect is always present but may be more noticeable at night because of the scheduled defrost.
• Warmer Ambient Temperature: If the area around the refrigerator (like a garage or un-air-conditioned utility room) has a higher ambient temperature at night than during the day (less common, but possible depending on your home's HVAC system and the climate), the refrigerator may struggle more when the defrost cycle is running.

3. ⚙️ Possible Component Issues (If the Rise is Excessive)

If the temperature rise is significant (e.g., above $10^\circ \text{F}$ and food is thawing), it could indicate a problem that is being exacerbated by the defrost cycle:

• Defrost System Malfunction: A fault in the defrost system (like a bad defrost thermostat or a misbehaving control board) could cause the defrost cycle to run too long or raise the temperature too high.
• Poor Door Seal (Gasket): A damaged or dirty door gasket allows warm, humid air to leak into the freezer.4 This warm air introduces more moisture, leading to faster and heavier frost buildup, which requires a longer and more intense defrost cycle to clear, resulting in a higher temperature spike. This problem may be more evident at night when the unit is in the passive, non-cooling phase of its operation.

✅ What to Check

1. Check the Defrost Cycle Time: If you're using a data-logging thermometer, check the duration of the temperature rise. A normal defrost cycle should last less than an hour.
2. Add Thermal Mass: If your freezer is sparse, place a few gallons of water or other dense, frozen items inside. The thermal mass will absorb the temporary heat from the defrost cycle and stabilize the internal temperature.
3. Inspect the Door Gasket: Check for tears, cracks, or dirt on the rubber seal around the freezer door. Clean the gasket thoroughly or replace it if damaged.