It really isn't. Most sensors suck. The reliable ones (i.e. the ones used in sleep labs) are uncomfortably tight (because they need to stay on to give useful readings). Most sensors stop being reliable as soon as any movement is involved. Even the really expensive ones in rings aren't all that.

And either way, SpO2 monitoring is not very useful for sleep apnea monitoring. The problem is not oxygen desaturation but rather arousals[1], which cannot be detected by looking at an SpO2 chart. Some people, even when sleeping with CPAP, have hundreds of arousals per night, wrecking their sleep quality, while their oxygen saturation remains perfectly fine.

[1] If sleep apnea is not treated at all, desaturation may be more prevalent, but even then arousals are a bigger problem unless saturation goes way down (below 85% or so).