Ok, waking isn't really the right word, but here's what's happening:
1) Use computer, all good
2) Go idle; screen switches off and computer locks after 5 minutes, all good
3) A few hours pass, still locked, all good
4) I return the next morning, sit down, start using the computer, enter password, unlock it, hurray, all good
5) ---> After a few seconds of using it, it goes to sleep... while I'm actively using it...
When 5 happens, it's very disruptive because it takes some time to regain access while I wait for sleep mode to fully settle in before I can unlock the computer again. Now I feel safe suggesting this shouldn't happen this way. But does anyone know why it happens exactly (it seems like the computer being locked is preventing sleep mode form engaging, so the OS queues the action until the PC is next unlocked, but that's pure speculation on my part) or a workaround for stopping it from doing this?