Sleep tech company Eight Sleep today said that it has raised $50 million in a strategic round led by Tether Investments at a valuation of $1.5 billion. This new round comes after the startup closed a $100 million round last August from investors such as HSG, Valor Equity Partners, Founders Fund, and Y Combinator. The startup, which sells smart mattress accessories that can track your sleep patterns and adjust temperatures as you sleep, didn’t disclose its valuation for the previous round, but it...