am currently working with a mare for "newbie horse owners". Between daily sunlight and bright yard lighting, the mare and her stablemate (a gelding), is under lights 24/7. I started working with this mare 21 days ago and she acted like the typical unbroke horse (easily excitable, pushy, hyper, etc.). I explained this away with the "this horse has never been handled" reasoning. This past week, I moved the mare to my facility. After being at my place for a couple of days, the mare settled down amazingly well, too well, I thought. My farrier asked me for assistance with the mare's barn buddy during his regular visit 2 days ago. The gelding (an 8yo 1/2 WB, 1/2 TB) acted the same way as the mare, wired as if on way too much caffeine. I asked if this horse acted like this when the family first adopted him and was told no, that a small child could ride him with no problems.

My question is: Does the 24/7 exposure to light (both daylight and artificial lighting) have a negative effect on a horse's system? I am familiar with manipulating a mare's reproduction cycle with artificial lighting, and would think all this daylight has this mare all screwed up. But what about the gelding? I looked through the Merck Veterinary Manual and did a Google search, but couldn't come up with anything.

I would like to advise the owners about this topic, and have a gut hunch they should reduce the amount of lighting in their yard. But, I couldn't find any hard evidence I could give them to read. Any input would be helpful.

Thanks!

Not long.  Once we got home they always got a few days to recover and getting a little R&R brought them right back.  The mind and body need to have down time which 24/7 lights does not allow.