Personally I am in the 'shut it down when not in use' camp. I use a Desktop usually, but I do the same even when I am using my laptop. You don't say what you are using.
If I am using my laptop and I know I may want to use it again in a short while, I think I would use Hibernate instead of Sleep. Sleep means the system is still ticking over and so many things can interrupt that sleep. Hibernate copies all open applications, files, and memory contents to a file called 'hyberfil.sys' file and saves it to the C drive, then it shuts down. But when you boot up again, instead of restarting everything the startup process looks for this hyberfil.sys file and loads the data from that. It can be quite a bit quicker.
You're right that this interruption is probably just normal updates. By normal I mean Windows Updates. You can control what Windows 7 does with these in the Control Panel > Windows Updates, and the "Change Settings" link on the left. In my Windows Update I have selected to "Check for updates but let me choose whether to download and install them". I do not know if this will solve the sleep problem, or if even this will interrupt it.
You could turn Windows Updates off before you send the computer to sleep, although that means extra steps before sleeping and extra steps to restore WU after wake up.
One other thing to note, it may be that other applications are also constantly scanning the internet for updates. One obvious candidate is your anti-virus/security software. However it is inordinately difficult to track down 'everything' that sets options to check the internet for updates.
Other than that, sleeping is going to be quite safe as long as the system is not doing anything eg file downloading through torrents, (something we generally frown on anyway), or no browser is open to an active and dynamic web site. if all applications are closed, and if the Firewall is continuously running, then the system is safe from hacking and malware. Of course, that may not be the case if malware is already present on the system and is attempting to 'phone home', for whatever reason.
I hope that helps. i suspect you will get many different answers here, 
Mark
My whole life I have been shutting down my computer when not in use. Even if I would just leave it for a half-hour, I treated it like a lightbulb. It's not in use, it goes off.
That is until I got my latest one, I found a discussion thread (on this site actually) that presented some cases for why sleeping might be a better alternative to a full shutdown. I noticed that the computer comes "alive" much more quickly from slumber than it takes to boot up, so... I figured why not give it a try. It's as silent as it is when it's fully off too. The only down side really is that damn blinking blue light that never goes off...
But lately I've noticed something weird... I'm getting ready for bed, I push the sleep button, and like... 15 minutes later, the computer just wakes up. I didn't touch the mouse or the keyboard or anything, he just wakes up. ONE time, I saw a pop-up in the lower right corner that said something about updates.
Another time I got sick of him waking up non-stop and tried to shut him down, but another popup appeared saying something like, "Would you like to wait for the following programs to finish <Updates>, or would you like to close all programs?"
So... it's probably just normal updates, but I am inherently suspicious of when my computer starts acting out of the ordinary and doesn't give me any warning! Is there a way I can track these upgrades as they are being downloaded? So I can be sure that that is what's really happening. And that my computer isn't really half-way through uploading some horrible malware while I sleep?
Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic