Stadium Capital, which owns roughly 9% of the Minneapolis, Minnesota-headquartered company, said Sleep Number is undervalued and is not performing to its potential. In the last 52 weeks, the stock price has dropped 42%. It closed at $25.96 on Tuesday after having topped $140 a share during the COVID-19 pandemic as mattress sales surged.

Pointing to "deficiencies in the boardroom", the investment firm wrote: "We urge you to collaborate with us on a Board refresh that includes a Stadium Capital representative and other new directors with relevant expertise."

The board "failed to oversee accretive capital allocation, effective forecasting and a culture of strong accountability for management," the letter said.

Stadium, based in New Canaan, Connecticut, criticized the board's decision to authorize more than $630 million in share repurchases at an average price of approximately $90 per share between 2020 and 2022.

The firm also argued the board erred by mistaking a pandemic-driven increase in sales as heightened demand for wellness technology.

A Sleep Number representative was not immediately available for comment.

Stadium Capital had previously asked Sleep Number to add three new directors, including one of the firm's principals, without success.

The standoff sets the stage for a potential proxy fight, with the company's window for nominating director candidates just two months away.

"The Board, as presently constituted, includes long-tenured members who have been part of questionable decisions and would likely not fare well in a contested election next year," the letter said.

Rival mattress companies including Tempur-Sealy and Purple Innovation have been the targets of successful activist campaigns.

(Reporting by Svea Herbst-Bayliss; Editing by Lincoln Feast.)

By Svea Herbst-Bayliss