why baby cannot use cradle when sleep?

VIBRATION:
vibration is the back and forth,side to side and up to down motion of the body that starts from and returns to the same reference position

EFFECTS OF VIBRATION:

• performance degradation
• tiring
• reduced comfort
• low back pain
• numbness 
• tingling sensation

motion of the cradle is a type of vibration.this motion can cause tiring and numbness to the baby in the cradle.sometimes,the baby might fall asleep because of the tiring effect of the cradle. not because they are sleepy.