Is it really illegal to lie down in a moving vehicle? (not if you're driving)

I heard for the first time the other day that it’s supposedly illegal to lean the passenger seat back and lie down while riding in a moving vehicle, even if you’re wearing your seatbelt. I had never heard that before. Is this true (in California)? Is it some kind of weird permutation of the general seatbelt laws?


Now if the phrase, “properly restrained by a safety belt,” is further defined in the law, then perhaps it is illegal to sit in a reclined position.

According to

I think the last part is the kicker. If your vehicle manual spells out that wearing a seat belt while sitting in a reclining position is ill-advised, then perhaps it is also ilegal as well under California law.

Of course, IANAL and I may be misinterpreting the above regs.