The person sorting a mixed pile is doing decisions as well as handling objects.

Sorting looks like a physical job and is mostly a mental one. Every item asks whether to keep it, where it belongs and who it belongs to, and each answer costs something. That is why an afternoon of tidying can leave you flatter than an afternoon of lifting. The tiredness comes from the choosing.

If you have a pile waiting, expect it to take more out of you than its size suggests. Decide a few rules in advance so you are not judging fresh each time. And let yourself stop before the judgement runs out rather than after.