Leave it on.
1) There is an automated process (which sometimes fails) that measures the speed capabilities of your line based on tweaking the speed upwards until the line drops and then backing off a bit. This can take up to 10 days to achieve a setting whereby the line is at it's fastest stable speed. You turning off the router at night can affect the results. (granted this is usually only done for the first 10 days of a new line, but sometimes the BRAS profile is amended mid-life).
2) Your wifi will be broadcasting on a defined channel (with overlaps) which is either manually set or automatically seeks to find the least congested airwaves. Since there are only 4 non-blocking channels that could be used, if you 'give up' your channel allocation at night, the chances are that another auto-roaming wifi access point in the vicinity will roam onto your preciously hoarded bandwidth, leaving your access point to either share with that when it turns back on (as auto roaming doesn't always happen without the AP having been rebooted) or you'll find your AP roams itself onto another channel which might not be compatible with your wifi card in your lappie (it does happen).
3) Having been out to install a router for a client an hour's drive away on Monday, only to be rung up just after i'd arrived home with them saying "there was a loud popping noise and i've got no internet again", there's something to be said for the resilience of components and power supplies when subjected to the on/off surges and heatcycles.
So, bottom line, leave it on....but I can understand why you wouldn't be affected if you turned it off.