As opposed to every 15 minutes through the night.

Could it be the email checking app is a power hog?

If the answer is maybe could it be dependent on the size of inboxes and/or IMAP vs. POP? There are reports that the email app is very slow on large inboxes - implying it's a CPU hog at processing them. That shouldn't affect fetching a few new mails but you never know, if it's doing something silly to compare remote and local inboxes. Just a thought.

We could with a tool that measures the cumulative system resources spent on different apps - CPU time, network activity etc. "time" does it for one process, but not cumulatively over many instances of the same process.