Related Clues:

• Red flag, e.g.
• Caffeinated?
• Trouble indicator
• Not dozing
• Not nodding
• 'Look out below!,' e.g.
• Awake and aware
• Having pricked ears
• Keen-eyed
• Like watchmen
• Paying attention
• Like sentries
• Like an excellent guard dog
• On top of things
• Not drowsy