You are describing the normal behavior of radio waves and how they propagate during the day versus night. Solar radiation affects the atmosphere much more during the day than at night and as a result, there is a marked change in the electrical charge of various layers of the atmosphere. At night, radio waves are more likely to skip or bounce off of the upper layers of the atmosphere. This can make TV and radio signals travel much farther at night. Like the visible weather, the atmosphere's effect on radio wave propagation varies for a variety of reasons, so long distance reception can be quite different one day to the next.
FWIW: Many AM radio broadcasters are required to change antenna radiation patterns and/or reduce power at night to account for the increase in range during nighttime hours. To my knowledge, that's not the case for TV stations typically.
What's the make and model of the antenna? (If you know.)
If you were starting from scratch, I would suggest a Winegard HD7698P or, a combination of an Antennas Direct 91XG and an Antennacraft Y10713... Either way a low noise amplifier such as the Antennas Direct PA-18 would outperform any of the RS amplifier products.
To get full use from a large antenna, some would suggest a rotator. If you have only one TV, you may find that an acceptable option, though there are other methods to deal with signals coming from several directions:
http://forum.tvfool.com/showthread.php?t=8275
http://forum.tvfool.com/showthread.php?t=2882
http://forum.tvfool.com/showthread.php?t=820