Eight Sleep, the startup behind the $2,999 mattress cover that puts both Elon Musk and Mark Zuckerberg to sleep, wants to stop being known for the celebrities who use it. Instead, the company is pursuing FDA clearance to detect and treat sleep apnea, a pivot it hopes will turn a bougie tech gadget into a business that insurers, not just billionaires, will pay for.Matteo Franceschetti, Eight Sleep’s cofounder and chief executive, told Fortune the company’s filing with the FDA is progressing “fairly smoothly.” The company began pursuing clearance with the FDA for its sleep apnea detection and mitigation technologies in…