Sentence: The hardest thing for me to do is sleep (to sleep, sleeping).

[Ched133]

I'm confused on how to express the following thoughts correctly
Which of the following sentence is correct? and why?

The hardest thing for me to do is SLEEP (verb as well as subject)
The hardest thing for me to do is TO SLEEP (Infinitive, can also be used as a subject)
The hardest thing for me to do is SLEEPING (Gerund which can also be used as a subject)

[Mister Micawber]

.
These are fine; 'to' is optional:

The hardest thing for me to do is SLEEP
The hardest thing for me to do is TO SLEEP

This is very awkward because 'to do' and 'sleeping' are not in the same form:

The hardest thing for me to do is SLEEPING

An editor would correct it to one of the first two sentences or to:

The hardest thing for me is SLEEPING
.
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Native English teacher at Mister Micawber's