This is where photos would help. I don't know what your set up looks like.
How high are the current roosts? What breeds do you have? For most breeds getting UP onto roosts isn't an issue, getting down might be, depending on landing space, as chickens aren't the most graceful landing.
Is this a traditional heat lamp that gives off light as well as heat? If yes, I think she took advantage of the fact that she can see easily and went to her sleeping spot of choice.
You have 12 hens and 2 bars measuring 4' long? Optimally you'd need another 4' span somewhere, it's really not...