close to the energy threshold it almost always crosses it when the temperature in your home drops in the overnight hours.

https://support.getroost.com/hc/en-us/articles/208015818-Why-do-smoke-alarms-with-common-9V-batteries-always-start-chirping-in-the-middle-of-the-night-?mobile_site=true#:~:text=Most%20smoke%20alarms%20operate%20on,alarm%20warns%20us%20by%20chirping.&text=This%20causes%20the%20battery%20to,battery's%20power%20output%20also%20drops.