It doesn't require much light to affect chickens and their hormones. 24/7 light will cause them the be hormonally active without the typical winter break. This is what chickens in commercial egg operations endure.
You can do it, but I suggest you think about whether this is really what you want for your chickens. A day/night light cycle is definitely healthier and more natural, but it's your call.
As to why you're putting up a light, if it's for security, a motion light might be a better alternative. Also, if you're doing it to keep predators away, it won't make any difference other than to make it easy for the predator to see what they're doing.