To some extent a good faith effort is in order, but someone who is perfectly capable of doing 99% of the job shouldn't be expected to respond to things at 3am unless they willingly do so for extra pay.
I for one am perfectly capable of advanced software engineering work but I am NOT capable of compromising my sleep without causing serious heart arrythmias. Some other person might have to deal with a baby. Some other person might sleep early and do meditation at that hour because it helps prevent panic attacks during the day. Everyone is different, and personal time and rest time should be respected. There is so much behind the curtain of off-work time that employers don't see, and don't need to see, they just need to respect that time.
I for one am perfectly capable of advanced software engineering work but I am NOT capable of compromising my sleep without causing serious heart arrythmias. Some other person might have to deal with a baby. Some other person might sleep early and do meditation at that hour because it helps prevent panic attacks during the day. Everyone is different, and personal time and rest time should be respected. There is so much behind the curtain of off-work time that employers don't see, and don't need to see, they just need to respect that time.