A Jewish persons day starts out at nightfall. At the changes of the watches, which is the separation between day and night. These time periods of change which happen throughout the twenty four hour period have tremendous spiritual ramifications due to their depth. Therefore, we must not ignore its significance and utilize this time wisely.
If someone wanted to sneak through a heavily guarded area, they should do so at the time of the switching of the guards. At this time, they might be able to even walk right through to the King without interference since the guards are busy changing shifts and occupied with many things.
The changing of the watches is a busy time period in shamayim as well as in this earthly realm. People are arising from sleep, running to & from work, traveling to Synagogue or school and women are cooking the main courses of food for the day. It is very easy to let this opportune time slip away without being spiritually productive. Therefore, we must set up our daily schedule to make this time productive and to be occupied in mitzvos and joy.