Now that extended sitting is thought inadvisable, would it be better to lie down periodically to minimize the sitting time?

Better to Walk. Being inactive is not advisable. Sitting and laying down are for that matter identical. Walking and exercising periodically have great health benefits by improving circulation and muscle strength.

Related Questions

Now that extended sitting is thought inadvisable, would it be better to lie down periodically, to minimize the sitting time?

How about walking. I am not sure what you mean by extended sitting time. To stay healthy walking in between activities like watching tv is beneficial for health.
Sitting is Bad. I really believe that the amount one sits is just another indication of a sedentary lifestyle aka "couch potato" laying down is unlikely to produce an improvement. Getting up and walking up/down some stairs or around the block or getting on a treadmill is more adviseable.
Stand up. Ot would seem better to stand up, stretch, move one's arms and legs. You don't want a problem with deep vein thrombosis, blood clots in the legs.