Related Clues:

• Ready to jump, perhaps
• On guard
• Wary
• Whistle, maybe
• At the ready
• Far from sleepy
• Bright-eyed and bushy-tailed
• Advance warning
• Watching
• Quick on the uptake
• Attentive
• Like an insomniac
• With ears pricked
• Not nodding
• Red flag, e.g.