Using Sleep AND LowPower.h
There is no 5 or 10 second sleep. You can't just arbitrarily choose how long to sleep for. The watchdog timer only has specific periods (defined by dividing the clock by a set amount) that it can sleep for:

enum period_t
{
    SLEEP_15MS,
    SLEEP_30MS,
    SLEEP_60MS,
    SLEEP_120MS,
    SLEEP_250MS,
    SLEEP_500MS,
    SLEEP_1S,
    SLEEP_2S,
    SLEEP_4S,
    SLEEP_8S,
    SLEEP_FOREVER
};


To sleep for 5 seconds you would need to sleep first for 4 seconds, then wake up, and then sleep for a further 1 second after that. For 10 seconds sleep for 8 seconds followed by 2 seconds.

Note that they aren't exact values, however, since the watchdog oscillator is very crude and varies wildly in frequency with both temperature and power supply voltage.