AM stations have to reduce power at night per FCC regulations
http://www.fcc.gov/encyclopedia/why-am-radio-stations-must-reduce-power-change-operations-or-cease-broadcasting-night
I've noticed, listening to AM station, that at about 6PM signal rapidly dropped (more noise). Perhaps there is a quiet period of few hours after 6PM when stations already reduced power but propagation is still not as good as it is at night.
http://www.fcc.gov/encyclopedia/why-am-radio-stations-must-reduce-power-change-operations-or-cease-broadcasting-night
I've noticed, listening to AM station, that at about 6PM signal rapidly dropped (more noise). Perhaps there is a quiet period of few hours after 6PM when stations already reduced power but propagation is still not as good as it is at night.