Firstly, there's a night time setting, that will push battles up to 8 hours ahead of the attack time. I.E. your night time is set from 12 AM - 8 AM, the attack comes at 4 AM, it is instead pushed to 8 AM.
Secondly, you can hire ANYBODY for your battles, so for example, you could hire EU players for a battle while you're at work, that's pretty much prime time for them.
Thirdly, considering they'll ALSO be NA, that's going to be a disadvantageous time for anybody, maybe not themselves, but a lot of NA players.
Finally, a lot of the current NA vs NA battles are actually EU players in our lands, so this is what could be happening to you.
I'd recommend, if you're the defenders, and have actual gear to defend, making posts on the merc recruitment thread - people are always glad to support peaceful traders in defense.