Base Layers

I always find 200 weight merino wool to be perfect for cooler/cold weather. However, 200 is on the light end of the midweight range, so since you sleep cold, you'll probably want a true midweight at ~ 260 wt.

Smartwool, icebreaker, ibex and Stoic make some good pieces. I have and use pieces from all for brands. I find that ibex and Smartwool tend to be the softest/least "itchy", not that any of them are particularly itchy.. And I'm really sensitive to that kind of thing.

You can also wear a light fleece to sleep or a puffy.

Way too open question, probably, as it depend on you, how much you are willing to pay, what you like, and so on.

I personally am fully into slight overkill on the bag for a given situation. It you already have one that is slight overkill, then that is the most "economical" sleep solution in terms of weight and everything else. But I'd personally suggest a merino wool hoody. Synthetic is ok too. Wear that over your innermost layer, and use both hoods (hoody and outer jacket). It will have the advantage that it will be a great layer for hiking as well. Full or half zip. Weight going up into expedition weight if you need that much. But you will have much more flexibility with stuff you can also wear while hiking. Vests are great for sleeping and hiking too, and provide a layer right where you need it most. In the end you will have to try things.

Since I live in the area, I can tell you what I would take if I went out tomorrow night.
I'd be ready to sleep in all of it, but hope it would usually not be necessary to do so.

Patagonia sikweight merino crew
Silkweight or better long underwear bottoms
Ibex indie merino hoodie
Houdini windshirt
Montbell UL down parka
WM Hilite (35 d.) Bag. Or maybe I'd just take a beefier bag since it would be only a few oz more. You can always take layers off, but once you are out of insulation….

This would probably be good down to 25 deg. winter in high mojave > 2000 ft elevation, or coastal range with super high humidity and a cold snap like last week.

del

More down in you bag IS much lighter, but that is a great suggestion. Makes me want to turn off the heat and put one on before I go to bed.

They have some merino blend layers for really cheap, I got some leggings for 18 dollars I think.

A baselayer that is really warm, will be ridiculously heavy.
THat is why you buy the correct bag for the temp, its always much lighter.

Usually baselayers are ultra thin, to wick and be able to hike it. These can weigh a total of 7-8 oz for top and bottom. If you start wanting stuff that is actually thick and warm, you may reach 16 oz or more. And it doesnt do what 2 oz of additional down would.

I just picked up a thin long sleeve Adidas Climilite at Ross in Cat City(southern cal) for under $30. 8.4 ounces
Layered on top of a 10 ounce Mountain Khaki merino wool shirt for maximum warmth, or underneath as a wicking layer, it works well. Weather permitting I prefer cotton underneath giving me a third thin layer. A down vest(Lukesultrilite, 4.8 ounces) when temps really drop.
The Mountain Khaki shirt is ok but overpriced. ("From Cheyenne Wyoming", made in China) Ross had merino wool golf sweaters for $25 that were probably just as good. They also had a number of hoodies almost as nice as the R1 at half the price.

While a heavier bag is your best bet, not everybody likes being confined. I like to sit up and still be warm. That takes a clothing.

A baselayer that is really warm, will be ridiculously heavy.
THat is why you buy the correct bag for the temp, its always much lighter.

Usually baselayers are ultra thin, to wick and be able to hike it. These can weigh a total of 7-8 oz for top and bottom. If you start wanting stuff that is actually thick and warm, you may reach 16 oz or more. And it doesnt do what 2 oz of additional down would.

+1

Relying on base layers as insulation renders them much less useful as part of a layering system. If you can't comfortably layer it under another couple layers of clothes then you might as well just buy some insulating garments like a down sweater, etc…

Thanks for all of your insights. I have been backpacking for a long time now, but have just started focusing on finding lighter gear and "backpacking smarter" Has anyone used a sleeping bag liner, would this be a good substitue for a warmer base layer right now my layering system is pretty basic: From top to bottom.

Wool socks by smart wool
stoic 200 merino wool long under wear
stoic merino 150 crew top
REI pants they are basic nylon that can convert into shorts
Patagonia R1
Sierra Designs Gnar down jacket w/ hood
SIerra Designs Hurricane Rain Jacket (rarely used)
Marmot beanie

I feel like this is a pretty sufficient for the weather I am dealing with but I still get cold at night, that's why I figured I'd maybe update
the base layers. But maybe a down vest could be a better alternative.

"Patagonia R1"

Since you've already got a base layer, a down jacket, and you're backpacking in dry climates, you might consider dropping the R1(Hoody, I presume) and replacing it with a Montbell Ex Light Down Jacket. It would be much warmer, layer well in your sleeping bag, and save you ~5 oz in the process for a size medium. I use it as my second layer over a lightweight base for 3 season Sierra hiking, in and out of the sleeping bag.