Possible Clues:

• Late sleeper is close to Welsh lake
• One active after dark
• Person who likes to be active late into the evening
• Is one wise to avoid early retirement?
• One staying up close to outside of wall
• One preferring to work when it's dark, with long shifts
• No 'early-to-bed' type
• Nocturnal human (with a bird)
• One working late
• Hooter not heard in the daytime?