I 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!

You've made a very interesting observation. Some horses and their behaviors can be very reactive to light. But the general behaviors you're concerned with don't seem to fall into any published clinical evaluations of photoic sensitivity. Other vectors could also include behavioral reactions to specific sounds or smells at the offending barn or lot.

The amount of artificial light they are exposed to during normal hours of darkness is not very well defined in your question. What would be excessive? I have mercury vapor security lights at two corners of my property. Sometimes the horses will sleep directly under them. Other times they will sleep in areas the light doesn't reach. But they haven't shown any adverse reaction to the lights.

I'd recommend experimenting by putting a fly mask on them at night, darkening and diffusing the artificial light. If that doesn't work, it might be time to investigate other sources of agitation; sounds and smells. And don't forget watching their grain ration. Too much protien in their diet can cause the exact same behavior you describe. ~FH

The lights at this property come on at dusk and go off at dawn.  According to the estate caretaker, the lights are full blown security lighting, not mercury vapor lights.  I live nearby and the lights are on all night and probably violate our "night skies" ordinances in the neighborhood.  I liken the lighting to a bright parking lot.

When I moved the mare to my facility, the owner provided hay for me to take with me, so the diet didn't change.  Both of their horses were taken off their grain rotation 2 months ago, at the suggestion of the vet.

One piece of the puzzle I neglected to post is the part that REALLY got me thinking.  The owners were concerned the gelding would be "lonley" and asked if I would bring one of my mares to their barn for a couple of days.  I agreed, and took my 2.5yo QH project mare over.  Now, she's the most easygoing mare of my bunch and is the type that just "goes with the flow".  After the 2nd day of being at the "new" barn, she also exhibited the same characteristics as the other 2 horses, which is TOTALLY out of character for her.  Again, no change in diet, as I provided my own hay for her feed.  I brought her home after 2 days.  She wouldn't load in the trailer, scrambled in the trailer on the way home and blew out of the back as soon as I opened the door.  For a couple of days, I was back at square one with this mare before she mellowed out.  She's still not quite right after 3 days at home, but well on her way to "recovery".

If there are some sort of sounds or smells on the property that are irritating these horses, wouldn't they become accustomed to them after some time?  The gelding has lived at the property for 3 years and the mare I am working with has lived there for 2.

 I dont have any studies only first hand experience of it, yes it does change them...

 I have seen similar behaviour problems.

They dont need any more than 16 hours of light a day including lights in the stables and usually that is only for breeding trigger or triggering coat changes for showing. Its completely unnessassary to have lights on 24/7 and would be a complete brain fry for the horses as the light changes body functions using the number of hours of light in the day as the switch for shedding or not.

 urgh I just cannot fathom sleeping with lights on myself for that long let alone having it effect my body functions as well

Yes, I totally agree. A vet on another forum has suggested the light doesn't have any effect on horse behavior and noted that horses in the extreme north live in upwards of 20 hours of light.  But, he couldn't provide any studies that would prove otherwise.  My argument with that correlation is that at least northern horses get 4 hours of nighttime.  It would HAVE to be better than 24/7 daylight.  But, I haven't been able to find any equine studies that deal with excessive light.  I think next option is to look for human studies and try to come up with a reasonable explanation for this mare's owner.

As a side note, the mare has now been down at my barn for 1 weeks and the transformation is remarkable.  She's lost that "freaked out" look in her eye and no longer looks at you as if she's mad at the world.

I am certainly no expert but every other animal/creature that I have ever read care manuals on has required some time of darkness.  I don't see why horses would be any different especially considering the amount of artificial light you say they are under all night long.  Is there not a way that the horses could be put in a shelter or a run in over night?

I would also wonder about how they were being treated.  Our horses are very in tune to the emotional environment.  You mentioned that these horses had been in the same place for 3 and 2 yrs, but you described the owners as "newbies".  Personally, I would think that if they act one way for you and another for their owners that maybe the owners need some "additional training".  Just a thought.  Good luck solving the mystery.