。
I have to sleep.
Literal
I [topic-は] sleep-[if-not-なければ] become-not [polite-ません].
The classic ~なければなりません construction expressing obligation, literally 'if [I] don't sleep, it will not do.' This double-negative-via-conditional is the standard polite way to say 'must.' The plain version is ~なきゃいけない or ~なければならない. 私は is usually unnecessary in this context but lends explicitness or contrast.