Japan Rail Pass - Overnight trains

Hello, If I take an overnight train at 11pm on a certain date, and this is the first trip of my JR pass, does it need to be activated from the evening I get on, or can i have it activate the next day which the journey is mostly taking place on. ie. overnight train depart tokyo for kanazawa 11pm 14th june, can the rail pass be activated from 15th only? I know you can do this with eurail passes and was wondering if this is the same. Shame to lose a whole days value for jus the beginning of the train ride.

I'm 95% that you're going to have to activate the pass the day you actually board the train. If any of the station employees check your pass before going in through the turnstiles, they're not even going to let you in since it's technically not valid...

Everything before midnight will not be covered. You will have to pay for it separately.

The night train you mentioned leaves at ueno at 23:33 every night. You can buy a train ticket at 450yen from Ueno to Omiya. The night train will stop by Omiya at 23:59 and leave at 00:01. The extra will only cost you 450yen.

Overnight trains are not covered by JR Passes, those are different lines. Check this website for a guide from point to point and time and date http://www.hyperdia.com/cgi-english/hyperWeb.cgi

out of curiosity, how do you end up at Itami at the start of your trip? you have JR passes therefore live outside Japan, right?. foreign flights usually land at Kansai or Kobe airport, not Itami, but I could be wrong.