Mt. San Jacinto Outdoor Recreation

For colder nights I was wondering has anyone here used a nested sleeping bag setup? Example: two 10-30degree ultra light bags nested withing eachother vs one 0 degree bag.

I see one advantage as nested you can use a single bag in warmer months vs the 0 bag which wouldn't be ideal for the warmer nights.

Haven't tried it myself, but I've heard a lot about "modular" sleeping systems. The only thing I'd be worried about is the outer bag compressing the insulation of the inner bag, particularly with down. I guess if the outer bag were a larger, more capacious bag you could eliminate a lot of the compression.

That compression issue is why I chose a Big Agnes bag this year.
It's got no insulation on the bottom and depends upon the air mattress (which goes in a sleeve in the bag) for bottom insulation.

I haven't spent the might colder than the teens, so I'm not up to nesting bags yet.

Another option is to use a light bag and wear clothing to enhance your insulation needs based on the outside temp. I do this with a 30F down bag and have been able to keep warm in 15F weather.

Two limitation on this:
A). You've got to have a bag with enough inside room so you're not compressing the down when you get inside in with a few layers of clothes. Look at a bag's specs, specifically the inside circumfrence mesure, to find the bigger ones.

B). Don't try this with a bag advertised as 'ultralight"; they tend to be cut extremely narrow (to help limit weight).

Also, if you're going to in a tent overnight with other people, everyone helps to contribute to inteior warmth.

Almost forty years ago I bought an Eddie Bauer 1 pound inner bag that could be used as a lightweight summer bag. Had no zipper and no baffles (sewn-through construction) and unfortunately no hood. I bought it to convert my three season bag to a winter bag. They claimed it would lower the rating by 25 degrees and worked satisfactory. The advantage was I could spend little and have a warm winter bag and a very light summer bag. The down side (no pun intended) was that the resulting winter bag was a bit heavy (~5 pounds). A few years later I bought a SnowLion Super Light that was rated almost a winter bag by making it really slender and as Jim mentioned, when I tried to use my inner bag I was really packed in tight and not able to move around at all...a terrible way to sleep.
I did sew on a hood to the inner bag and still use it today (if you take care of good down it will last forever).
I now own seven sleeping bags including a -20 down and a -30 synthetic...no such thing as a bag good for all situations but there are ways you can increase the warmth of a bag such as:
Good insulation under you
Keep it dry
Unstuff when you get to camp and allow it to "fluff"
Don't wear damp clothing inside
A water bottle with warm liquid in the foot section, make sure it won't leak
Have a good meal
Be in a tent, bivy sack, igloo, wind break (point is out of the wind)
Don't hike till your exhausted and expect to sleep warm
and more if I had time to think

Thanks for all the responses. I'll look into the sizes of the bags for the outer so the insulation is not all compressed. And it's a good thing to know ultra light are cut thinner.