The Commons in YSN West Campus is available 24/7. After 6pm and before 7am Monday through Friday you will need your ID to access. Weekend access also requires your ID.
The Medical Library may also be handy for you. Find hours.
Also for your convenience there are many other places across the Yale campus to study. For “after hours” study in these locations you may need to check whether your ID will let you into the study space. If it doesn’t, you may be able to request access.