Related Clues:

• Caffeinated?
• Trouble indicator
• Not dozing
• Not nodding
• 'Look out below!,' e.g.
• Utter '...or else!'
• Awake and aware
• Having pricked ears
• Keen-eyed
• Like watchmen
• Paying attention
• Admonish
• Cry 'Heads up!' to
• Like sentries
• Like an excellent guard dog