POSIX 7
First find the function: http://pubs.opengroup.org/onlinepubs/9699919799/functions/nanosleep.html
That contains a link to a time.h, which as a header should be where structs are defined:
The header shall declare the timespec structure, which shall > include at least the following members:
time_t tv_sec Seconds. long tv_nsec Nanoseconds.
man 2 nanosleep
Pseudo-official glibc docs which you should always check for syscalls:
struct timespec {
time_t tv_sec; /* seconds */
long tv_nsec; /* nanoseconds */
};