Why is a smoke alarm chirping at specific times of the day?
I have noticed this same phenomenon before, and the only explanation I can think of is that early morning hours are typically the coldest hours of the day in a house.
Smoke alarms operate with a battery (sometimes only battery, sometimes the battery acts as a backup for a hardwired power source). The battery voltage is monitored, and when the voltage drops below a threshold (indicating the battery is nearly dead), the alarm starts the low battery chirp to make you put a new battery in. Battery voltage is affected by temperature, so when the temperature drops a bit, and the battery is right at the threshold between OK and low, this temperature drop might cause the voltage to drop enough to trigger the chirp. Once the battery warms up a bit, the chirp might stop.
After a few days or weeks of this, you will probably eventually get a constant chirp, and then the chirp will stop because the battery is dead (at least if the battery is the only power source).