问题标题: 酷町堂：奇怪……

#include<bits/stdc++.h>
#include<windows.h>
using namespace std;
int main(){
    cout<<"HI!初次见面，别来无恙\n";
    sleep(500);
    cout<<"我帅不帅？\n";
    sleep(500);
    cout<<"帅（Y）不帅（X）啊呸（A）臭不要face（F）（大写的哦）\n";
    char a;
    cin>>a;
    if(a=='Y'){
        cout<<"EN,诚实的小孩";
        sleep(1000);
    }
    else if(a=='X'){
        cout<<"EN,不诚实的小孩";
        sleep(1000);
    }
    else if(a=='A'){
        cout<<"EN,啊呸";
        sleep(1000);
    }
    else if(a=='F'){
        cout<<"EN,你也是！！！";
        sleep(1000);
    }
    else{
        cout<<"看不懂中文的走开！";
        sleep(500);
        return 0;
    }
    system("cls");
    return 0;
}

为什么第六行报错？说 [Error] 'sleep' was not declared in this scope

？？？