ATMega32u4 SLEEP instruction
SLEEP is an assembly instruction.  You can execute assembly with GCC's Inline Assembly facility.

In short:

asm volatile("SLEEP");


will do what you want.

The avr/sleep.h file has sleep_cpu() defined as:

#define sleep_cpu()                              \
do {                                             \
  __asm__ __volatile__ ( "sleep" "\n\t" :: );    \
} while(0)


Which is essentially the same thing - just wrapped in a block to isolate it from the rest of your code somewhat.