My guess, it's a mix of subtle changes in temperature, external noise patterns and light that are detected for the rough estimation, and then is when stress kic

My guess, it's a mix of subtle changes in temperature, external noise patterns and light that are detected for the rough estimation, and then is when stress kicks in, and we do a lot of micro time checks with eyes slightly open that we forget, personally I remember checking the time semi consciously like 100 times, before "accurately" stopping it just one minute before, but other times I experience opening the eyes one second before the alarm rings and it feels so amazing that I suspect I forget everything else.