In terms of general health and battery life, is it better to leave the n900 on standby or turn it off when not in use for long peroids of time (e.g. sleeping)

Unless you're planning on sleeping for more then a couple days, I'd go with standby

not sure about N900 (which I have only had for a week). For prior generations N8x0 it was better to leave on than turn off, as the boot ate battery. With N900, not so sure. 3G seems to suck energy, but if you are at home and connected to WIFI, battery drain is probably not that bad.
I have no hard info, as I keep the N900 plugged in overnight.

Standby. I think it might even use more power booting up than being on standby for a night.

I put it in offline mode and leave it on standby overnight.

I'd vote for standby too. If you don't need it to be connected, switch it to offline mode. It barely uses any battery overnight.

If it uses more battery power to shut down and restart than to leave it switched on overnight, it should be easy enough to test it.

1
a) Fully charge the N900
b) Leave it on standby/offline until it dies (This is the hard part as you're supposed to simulate that you're sleeping so you should be doing absolutely nothing on it for days!!)
c) Record how long time it lasted. This will be X hours.

2
a) Fully charge the N900
b) Switch it on and off as many times as you can until it will not power on because of lack of battery power.
c) Record how many times you could do that. This will be Y times.

Now divide X/Y. This will give you how many hours you can sleep per night. If it is less than you are comfortable with, switch off the phone, otherwise leave it on standby.

I used to do that. There's this annoying bug though that brings up a "do you wnat to leave offline mode" message soon after you enter offline mode... and then, when you really want to go online again in the morning, claims it doesn't find any connections (even though it's about to connect to the wifi the very moment it says it doesn't find it).

im very annoyed about this sloppiness and therefore leave it on - usually charging overnight.

I left it on standby last night for 10 hours, the battery hardly moved so that's good. I have wi-fi set to manual connection and never use 3G so I think standby is the best way.

I've never seen this, I just go to "disconnected" mode every night and "connected" in the morning. Very convenient. I don't see any messages. Is the 'offline' mode you mention something different from what you see in the menu when you hit the power button? Are we talking about the same function?