Related Clues:

• Like Argus
• It may do a number on your slumber
• Warning bell
• Ready to react, perhaps
• Eye opener, of a sort
• Make aware
• Wide awake
• Red light
• Give notice
• Chili-hotness measure
• Perturbation
• On one's guard
• Like a good guard dog
• Clock partner
• Snooze, for one