If you run man man
you will see the following information in the man page:
1 Executable programs or shell commands
2 System calls (functions provided by the kernel)
3 Library calls (functions within program libraries)
4 Special files (usually found in /dev)
5 File formats and conventions eg /etc/passwd
6 Games
7 Miscellaneous (including macro packages and conven‐
tions), e.g. man(7), groff(7)
8 System administration commands (usually only for root)
9 Kernel routines [Non standard]
Some names are associated with multiple entries, eg on my system 'sleep' has an entry in section 1 and an entry in section 3. You can specify the one you want with e.g.
man 3 sleep
Sometimes I just guess with
man -a sleep
which displays each entry associated with sleep in turn. I just go through them until I find the one I want. You can also try
man -k sleep
to get a slightly bigger listing of pages involving the term 'sleep'