Based in London, Cayton will work with the senior global team at Lightspeed Research and report to Day.
Cayton joined Lightspeed Research in 2003 as senior vice president of business development, EMEA, where he was integral in leading the region to profitability.
In January 2009, he was appointed global sales operations director while retaining his business development role.
In both his global and regional roles, Cayton was responsible for sales operations training and development.
He also served as a director on the Lightspeed Research board.
Day said: "Andrew was a natural choice for the CEO role. Over the past six years, he has played an integral role in defining and executing our strategy, which contributed to the rapid growth of Lightspeed Research's EMEA business."
Prior to joining Lightspeed Research, Cayton was the head of sales at Harte-Hanks, where he promoted the company's insight and marketing services to companies within the IT, internet, telecoms and finance industries across Europe.
Cayton said: "This is a particularly exciting time to be part of Lightspeed Research. With the recent merger of the TNS 6th dimension panel business into Lightspeed Research, we are now in a position to build the preeminent global online access panel business.
"I'm hugely excited at the opportunity of integrating the TNS panel business and talent, continuing to build our offer, and providing high quality research solutions to our clients."