Historical clue formulations

• Trips in the dark — 2020-01-17
• All-nighters? — 2018-01-19
• Overnight flights — 2017-10-17
• They leave at night — 2015-09-18
• Two-day trips, sometimes — 2012-09-02
• They might depart at midnight — 2011-03-10
• Result of lack of sleep, maybe — 2006-03-10
• Overnight flights — 2006-01-16
• Fly-by-nights? — 2003-11-01
• Signs of insomnia, maybe — 1998-06-06