I really think the Main Library at least should be open 24/7 - it is absolutely ridiculous that this is not the case at present. I think that Durham is far behind other universities (and this is sad to think so) in the UK in realising that it is up to individuals to judge how much time they spend studying, and moreover the onus is on the University to provide the resources to do so.
I do not see how at present the Main Library can operate with no staffed services after 8pm and yet this cannot be extended beyond midnight. I know that I would personally benefit by a significant amount if the library were to be open 24/7 - particularly in exam term.
Indeed, at present the University Library opening hours are actively detrimental to study. If I were to stay until closing time and then be forced to leave, my train of thought, progress and engagement with my work would be affected. Having to leave at midnight and return home, breaking study, is a far worse situation than having the library open constantly and being able to leave whenever one wished. Having time limits imposed on study is not productive and I don't think it would be too much of a hassle to implement longer opening hours.
I particularly rely on the library as part of my University education. Being an Arts student I have a minimal amount of contact time and so my main resource is the Library. I think that even if a minority of students would benefit from 24/7 opening it should still be enacted. Longer opening hours would either have a positive or a neutral effect - I don't think anyone or anything could suffer as a result from it.