Booking a trip can boost happiness. United Airlines is adding joy with bonus miles.

It's National Plan for Vacation Day, which coincides with some of the darkest, coldest days of winter. To help travelers observe – and possibly boost their moods – United Airlines is running a promotion that offers bonus frequent flyer miles to travelers who book flights to select international destinations.

The airline also partnered with Laurie Santos, a cognitive scientist and host of "The Happiness Lab" podcast, to learn more about why travel can be so uplifting.

“Planning a vacation can give you an unexpected happiness bump because experiences matter for our well-being more than we expect,” Santos said in a statement. “When you book a vacation, you experience the reward of anticipating a positive experience. The happiness boost we get from travel can last long after you book your ticket - from the moments you happily anticipate your getaway, to taking the fun trip itself, to the times you recall trip high points with friends once you get back home.”

A Jan. 8-9 study conducted by YouGov and published by United showed that about 70% of the 1,323 Americans surveyed said booking a vacation made them happy, and nearly the same number said January was a month when they especially felt like they needed something to look forward to.

United's Plan for Vacation offer

If you book a United economy flight to or from select cities on Tuesday for travel between Wednesday and Nov. 15, you'll be eligible for bonus miles at a rate of two times the fare for the qualifying flights.

Here are the cities covered by the promotion:

• Malaga, Spain

• Bilbao, Spain

• Cebu, Philippines

• Christchurch, New Zealand

• Dubrovnik, Croatia

• Dubai, U.A.E.

• Faro, Portugal

• Madeira, Portugal

• Nuuk, Greenland

• Hong Kong

• Kaohsiung, Taiwan

• Osaka, Japan

• Melbourne, Australia

• Manila, Philippines

• Nagoya, Japan

• Porto, Portugal

• Ponta Delgada, Portugal

• Beijing, China

• Palma de Mallorca, Spain

• Palermo, Italy

• Puerto Escondido, Mexico

• Marrakesh, Morocco

• Singapore

• Tenerife, Spain

• Ulaanbaatar, Mongolia

To qualify for bonus miles, the flights booked must be operated by United Airlines or United Express and booked directly through the airline's website or app. Basic economy tickets are not eligible for bonus miles under the promotion. See United's website for full terms and conditions of the offer.

Zach Wichter is a travel reporter and writes the Cruising Altitude column for USA TODAY. He is based in New York and you can reach him at zwichter@usatoday.com.

This article originally appeared on USA TODAY: United Airlines National Plan for Vacation Day bonus miles offer