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I've got a winter sleeping bag rated down to -40c but its heavy and it won't get that cold where I am going. The coldest it will be is +5 to -5.

I've another lightweight sleeping bag rated 5 degrees comfort and it weighs very little. I would rather take this bag and know how to get warmer if needs be by putting on more clothes for example, but this might make me sweat..

So any best practice recommendations for how to get warmer if the temperature does drop further?

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You covered it already—layers. This is actually a very common technique among ultralight backpackers. They will often intentionally bring a sleeping bag with 5-10ºF less insulation than needed and compensate by wearing all their clothing to bed. If you need to get warm fast, you can do some sit-ups to get your blood moving.

You only need to worry about sweating if you overinsulate. The form of insulation (sleeping bag or clothing) won't make a difference. In fact, having more layers gives you more flexibility so you can adjust your insulation level depending on the actual conditions. If you bring your -40 sleeping bag, you will overheat and sweat.

Source: Common knowledge, but available in Mike Clelland's Ultralight Backpackin' Tips among other places.

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    It is important not wear your 'day' cloths in the sleeping bag - they will be damp from sweat and cool you down. You can benefit by putting these cloths under you for insulation from the ground.
    – user5330
    Commented May 23, 2015 at 22:09
  • Thanks for the nice answer. The kind of insulation in terms of which kind of clothing you where can make a difference- for example cotton vs synthetic breathable fabric. At least thats what I've found.
    – Andrew Welch
    Commented May 24, 2015 at 14:20
  • Mattnz, that's true if you're going to bed directly after hiking. If you have sufficient time to dry out in camp, I've found it's a non-issue. If I'm hiking in a very rainy terrain, however, I will bring extra sleeping clothes since my hiking clothes will not have an opportunity to dry out.
    – Mitch
    Commented May 25, 2015 at 1:25
  • @Mitch: A few lucky people sweat less and manage there temperature well, so there day cloths are dry enough to get away with it. In sub zero temperatures, the rest of us risk a cold, miserable sleepless night - personally I perform better carrying an extra couple of hundred grams of cloths on a good nights sleep.
    – user5330
    Commented May 26, 2015 at 22:27
  • Oh, in sub zero fahrenheit temps I agree, I'll bring a change of base layers. I was talking about three season backpacking—OP stated a minimum overnight temp of 23º.
    – Mitch
    Commented May 27, 2015 at 23:39
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Aside from layering your clothes inside your bag, or layering your bag using a bag liner, it's also important to make sure you have a good ground sheet or sleeping pad that will insulate you against the ground and reflect your body heat back up. You lose a lot of body heat into the ground while sleeping, so at the very least, using a survival blanket as a ground sheet (shiny side up) will reflect some of that heat back up. You also lose a lot of heat through your head, so bring a toque or warm hat to wear while you sleep, this adds a surprising amount of warmth and comfort.

Eating will keep you warmer too. Deer spend a lot of time feeding in the middle of the night when it's cold out, not because they're hungry, but because it's fuel on the fire and keeps them warmer. Eating a good hot meal before bed will help your body generate more heat during the night. Emptying your bladder will also help, your body wastes a lot of energy keeping a full bladder of water warm.

Tricks for warming up in the middle of the night include preparing a thermos of hot soup or hot drink just before bed, and drinking it when you wake up cold. In the himalayas everyone carries a pee-bottle for when they have to pee in the middle of the night. Instead of getting out of their warm bag, they'll pee inside the bottle, then keep the warm pee-bottle in their bag until morning.

Personally, I pack a warmer bag, I sleep with a -10ºC bag in the summer time, if it's warm enough out I'll use it as a down comforter when I lay down at the end of the day, but I always end up zipped up inside it sometime during the night. Even in the middle of summer it still dips down close to freezing at night here in the rockies.

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    To clarify on the heat transfer a bit, most of your lost heat when sleeping on the ground is from conduction, not radiation. A survival blanket (aka space blanket) is good at reflecting radiation but will not help at all with conduction, as it's made of extremely thin plastic with a reflective coating. An insulating foam pad does effectively block conductive heat transfer. The thicker the pad, the better it insulates.
    – nhinkle
    Commented May 23, 2015 at 22:37
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    Instead of getting out of their warm bag, they'll pee inside the bottle - it is wise to practice this at home, with a washing machine nearby.
    – Nate Eldredge
    Commented May 24, 2015 at 0:54
  • @NateEldredge - Most people use a bottle that has a large enough mouth for you to "fit inside it", like a third Nalgene that's clearly marked as being your pee bottle, so as long as you have enough volume inside to hold everything you empty from your bladder, then you shouldn't have to worry about spills. Women may not have it as easy if they haven't practiced with their freshette or similar device.
    – ShemSeger
    Commented May 24, 2015 at 4:22
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    As sleeping mats tend to be bulky a lot of people carry a 3/4 length one. I find sticking the foot end of my sleeping bag inside my (empty) rucksack helps keep my feet warm when they're hanging off the end of the mat.
    – aucuparia
    Commented May 27, 2015 at 9:08
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Several good tips have already been posted; another to consider is a hot water bottle. I was on a trip for a mountaineering class recently and was surprised by how many people had never slept with a hot water bottle before! Use a solid bottle which won't leak and is designed to handle high temperatures, like a Nalgene bottle. If you're already carrying fuel and heating water to cook with, it's easy to heat a bit more and throw it in your bag before you go to sleep. Then it'll already be nice and toasty before you get in.

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I would like to also recommend a sleeping pad. They make a big difference for comfort on bumpy ground in addition to adding insulation. I think the closed cell pads work better than the inflatable ones. But, that's a personal choice.

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If you are car camping,you can take both. If not, a good down jacket that is really lightweight down. Wear a hat to add warmth.maybe socks too.down booties are another option to stay warm.

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