Fleece or thermal layers for summer

I think you'd regret it if the weather did turn nasty.  Unless you are moving fast and B and Bing!

I feel the cold once stopped.

If camping/bivvying,  I would be inclined to carry a light fleece 2-300g and a primaloft item - e.g Rab Generator. (the primaloft item would be part of my sleeping gear too)

Daniel,

If you're trying to cut down the weight against a 500g fleece and need to keep a good warm layer for colder weather / stops then you'd do well to look at synthetic insulated belay-style jackets or, perhaps, down midlayer. I've a Crux Halo that would be a great companion but there are plenty of brands and designs to choose from. Of course that would need to be kept dry so a dry-bag or robust plastic bag will be needed to carry it in and you'll need good rain gear to wear over the top but it's still the option I'd probably go for. My plan B would be a microfleece and lightweight primaloft gilet which should come in at less than 500g and give  more versatile layering options. 

Compression tops generally have a high lycra content and aren't particularly insulative.You'll save weight and bulk over a 500g fleece but if you do find yourself in cold weather it won't be as warm. 

Have a great time!

I've got a 300g down jacket. Maybe you could carry a really light fleece as well for when you're actually hiking and use the down jacket at camp and as a pillow which is what I do as a 3 season practice.

I take 2 jackets - one is a lightweight waterproof breathable (Marmot Precip) and the other is a very lightweight (100g) waterproof and very breathable dri-ducks. This brand is no longer available and has been superseded by froggtoggs. This way I stay very dry when I hike as I wear the light one under the heavier one in serious rain. Our annual rainfall gets up to 12 meters per annum in some parts of Westland, New Zealand.

With the 2 jackets, not only am I protected against rain but it cuts down the effect from wind. I seldom need another layer on top of my base layer (Mountain Hardwear Transition top) if I'm wearing these 2 rain jackets. When camping out, the dri-ducks is particularly soft to use for lying on e.g. on top of my pack which is under my legs, or under my head along with other sundry items if I'm opting to use the down jacket inside my sleeping bag for extra warmth.

At $10 per jacket (?5 quid) I bought 5 of them...