AQI being higher at night is fairly normal as I explain here:
Sleep mode just keeps the fan on low (and display off), so that it doesn’t disturb someone sleeping.
If LED display and fan speed going higher at night doesn’t matter for someone, I recommend basically always leaving it on auto. There are countless things (90% of which are not visible to our eyesight) that make a difference on the AQI, but auto will increase or decrease the speed based on the current needs.
Having low AQI is a really good thing though. 