Actually, separate sirens are used to indicate the onset of day and night (night - 3 short signals, day - 1 long one), so as soon as you hear and recognize a siren, you can safely turn it off and wait for the next one. But I realized that I also need to describe this in more detail at the beginning, especially because, as in life, the alarm signal (dark saturation) can be longer than the dark saturation time itself, so you probably did not recognize the second siren (daytime).