Atmega328, sleeping mode using "LowPower.h" => time as argument

I am trying to make my atmega sleeping for a specific time using "LowPower.h".
As you might know, the maximum is 8 seconds, so, I decided to improve and do something like that :
Putting the sleeping time as an argument of the function.
I did not put everuthing, which is not necessary, but the problem is located at the argument of the function from my char table when I compile it.

int INTERVALLE=30;//Minimum 10 secondes
int mult;
int i,j;
char sleeping_char[10][12]={"SLEEP_8S","SLEEP_4S","SLEEP_2S","SLEEP_1S","SLEEP_500MS",
"SLEEP_250MS","SLEEP_120MS","SLEEP_60MS","SLEEP_30MS","SLEEP_15MS"};

int sleeping_time[10]={8000,4000,2000,1000,500,250,120,60,30,15};
double old_time;

old_time=0;
      for (i=0;i<10 ; i++){
          mult=(((double)INTERVALLE*1000-temps-old_time)/sleeping_time[i]);
          for (j=1;j<=mult ; j++){
              LowPower.powerDown(sleeping_char[i], ADC_OFF, BOD_OFF);
          }
          old_time=old_time+mult*sleeping_time[i];
      }
      delay(((double)INTERVALLE*1000-temps-old_time));