Agree with Dreamcatcher that stalling is not *necessarily* the devil. Likewise, just because they're kept out in a paddock doesn't necessarily make it good. I disagree tho that horses have *evolved* to suit different management. No, in that manner, they're the same as they were 1000's of years ago. Aside from minor changes like particularly 'thin skinned' TBs who need rugging for eg. I am a 'never say never' sort of a person, who thinks that there are almost always exceptions to the 'rules' and that compromises don't necessarily mean problems.
BUT that there are SO many health *& wellbeing*(don't think that should be undervalued either) issues with keeping horses cooped up, esp 24/7, that you need to understand fully in order to consider fully. Just because a horse can survive living in a box, or for that matter a small, solitary paddock, doesn't mean that it's good for it. And if horses must be kept in such artificial & restrictive environs, then you need to ensure you compensate in a variety of ways, for what they'll be lacking in. Obviously exercise is one big one, but that's not the only important consideration.
Anyway, the other consideration is whether it's going to be temporary or long standing, and it sounds like this is a temporary arrangement for your horses. So if it will be changing for the better soon, while I'd still want to ensure they got out & about for at least a couple of hours EVERY day, I wouldn't stress too much about it either.
Agree that stalling part time, is not necessarily bad, esp on a mature horse that is worked regularity. I have to disagree that it is not a negative, when it comes to young growing horses
In the words of Dr Sid Gustavson,who is a vet that specializes in equine behavior and welfare, \horses have a great ability to adapt, but we must never exceed their ability to do so'
Horses are also creatures of habit, and that is really demonstrated by some PMU mares. Laws were passes that mares on line, had to have that harness for collecting urine, taken off , once a month at least, and those mares given some turn out.
Those mares would be waiting to go back into that barn.
Obviously, it is good for a horse to accept some stall time, esp if he is a show hrose,, or just in case he will ever need stall rest
Other then that, they are healthier (and enough research shows that, far as stable vises, ulcers, colic, hoof health, skeletal development ) and the fact just can't be disputed that horses, esp young growing hroses do best with full time turn out, needing only a wind break /shelter
If I had a show horse, that I rode reguarily, or that got a big turn out area during the day (not a little paddock), I would be fine with that horse stalled at night.
A young growing horse, esp with those problems-nope!