May be easier on horses and your tow rig durring summer month's but, harder on you! If your normal sleep habits are to sleep in the day and work all night you won't have any problems. If you like to sleep at night, your sleep cycle can get screwed up and you could fall asleep going down the road.

You also have to consider the possibility of break downs. Tire shops are not open at night and I'm sure a wrecker wouldn't mind comming out to get your rig at 3 am for double the price when your cam shaft position sensor fails. As you can tell I like to run at night and have experienced these problems. 

I'm sure it would effect them to an extent. There are all kinds of factors to add up and consider. Some things on that list would be, how far are you traveling and will they be able to rest a full day upon arrival or will you need them first thing? A 200 mile trip that can be done in 4-5 hours (thats a little on the slow side) and still give the horse and yourself time to rest before performing can easily be done. But, on the extreme side, you dont want to leave the house Friday night at 6pm, drive 500 miles and expect your horse to perform well at 10am the following morning. Consider them to be like you. If you need 5 hours rest after travel and before performing then they probably do to. 

You're right, there is a lot less traffic, but I am a rubbernecker.  I like to see the country I am driving through!  I also don't like to come up on unexpected curves without having enough time to slow down for the horses.  My husband was a truck driver for many years and he always preferred to drive at night.  I guess it boils down to personal preference.

Marla