Is it true that it is cheaper to book a flight during night time or is this a myth?
There is some logic that is followed that would make buying a ticket before midnight… somewhere.
GDS (global distribution systems) are the controllers of access to the inventory of airlines that OTA’s access in most cases. The GDS are set to time zones/markets on which the airline base price. So a NYC time zone versus a Ho Chi Minh City time zone can see differences in pricing, from same airline (assuming the market pricing is the same - which it is not… that is another story) as the Vietnam is a day ahead of NYC based on the dateline. This means that promotional fares are released earlier in Asian markets than in the USA. Ex. Seat sale from 1 November begins a day earlier in Asia, because it is still Oct 31 in the USA. It could be that the promo seats are gone by 8am NYC time, so if you buy in the middle of the night (which is day time in Asia) you may is a cheaper rate.
Likewise as fares are released at midnight, the early bird gets the worm and even local promotions (ex Only available in your market) get loaded for consumption.
OTA’s like Future.Travel make use of this time differential to book and ticket in various markets around the world. Many times it is in the middle of the night in a ‘local’ market. But ahead of the dateline deadline so the lower fares apply.
In very rare cases an airline may put up a 2–12 promo fare, sometimes in the middle of the night, but usually during local market business hours… or on a weekend when they know people are shopping.