--- a/src/queue.c
+++ b/src/queue.c
@@ -8,6 +8,7 @@
 #include "../include/queue.h"
 #include <errno.h>
 #include <time.h>
+#include <unistd.h> /* sleep() */
 
 #define RETRY_LOG_INTERVAL	600		/* Seconds */
 #define MAX_RETRIES	20				/* Max number of times to retry sending message */