diff --git a/unix/modtime.c b/unix/modtime.c
index 196b1506a5..6843238cf3 100644
--- a/unix/modtime.c
+++ b/unix/modtime.c
@@ -121,7 +121,9 @@ STATIC mp_obj_t mod_time_sleep(mp_obj_t arg) {
     tv.tv_sec = ipart;
     int res;
     while (1) {
+        MP_THREAD_GIL_EXIT();
         res = sleep_select(0, NULL, NULL, NULL, &tv);
+        MP_THREAD_GIL_ENTER();
         #if MICROPY_SELECT_REMAINING_TIME
         // TODO: This assumes Linux behavior of modifying tv to the remaining
         // time.
@@ -139,20 +141,26 @@ STATIC mp_obj_t mod_time_sleep(mp_obj_t arg) {
     RAISE_ERRNO(res, errno);
 #else
     // TODO: Handle EINTR
+    MP_THREAD_GIL_EXIT();
     sleep(mp_obj_get_int(arg));
+    MP_THREAD_GIL_ENTER();
 #endif
     return mp_const_none;
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(mod_time_sleep_obj, mod_time_sleep);
 
 STATIC mp_obj_t mod_time_sleep_ms(mp_obj_t arg) {
+    MP_THREAD_GIL_EXIT();
     usleep(mp_obj_get_int(arg) * 1000);
+    MP_THREAD_GIL_ENTER();
     return mp_const_none;
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(mod_time_sleep_ms_obj, mod_time_sleep_ms);
 
 STATIC mp_obj_t mod_time_sleep_us(mp_obj_t arg) {
+    MP_THREAD_GIL_EXIT();
     usleep(mp_obj_get_int(arg));
+    MP_THREAD_GIL_ENTER();
     return mp_const_none;
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(mod_time_sleep_us_obj, mod_time_sleep_us);