Is it considered unprofessional to send an email late at night?

It's not unprofessional unless you expect some sort of response before normal business hours the next day, or are trying to use it as an avenue of communication for an emergency (which would warrant a phone call rather than email after business hours.

From my experience - absolutely not. People work at all different hours and in countries around the world in different time zones. Communication never stops in today's world. Just don't expect a response late at night. In fact, always allow a reasonable amount of time for responses....and consider people may be on vacations, out sick, taking some time off, etc. I always leave an automated email response that goes out whenever I am out of the office for longer than a weekend.