Possible Clues:

• Burning the midnight oil
• Awake into the wee hours
• Pulling an all-nighter
• Burning the midnight oil, so to speak
• Still awake at 1 a.m., say
• Cramming for a test, maybe
• Cramming for a final, perhaps
• Watching infomercials, maybe
• Watching Letterman or Conan, say
• Watching Craig Ferguson, say