Cold nights early September?

[Paladin2]

Will I need long johns at night hiking the JMT from 8/23 to 9/15 if I have a 25-degree (actual comfort level) sleeping bag?

[chknfngrs]

I would have them handy just in case

[Miner]

I always have a light pair, even if I don't need them. When hiking in the high country there, I always assume I need to be prepared for 20F temperatures. The mornings can be cold in September and I often start hiking with them on before pulling them off later in the morning or when finally warming up on a big climb. I"ve also been snowed on in every month in the Sierra Nevada including July and August. I once accidentally brought the wrong sleeping bag one labor day weekend (40F instead of the 20F bag) and saw the temperature drop to 22F during my last night (was 30F the night before at a different location); having extra layers really made the cold just barely manageable where I could get some sleep.

[garlic08]

Good advice above and I agree, but it's probably more a comfort issue than a survival one. I thru-hiked the PCT with a couple who wore shorts the entire hike. I never heard them complain. They were pretty tough.

[HooKooDooKu]

I would recommend having a little something extra for comfort.
I just got back from a 3 week trip on the JMT. I took my 32º sleeping bag, and on a few nights, I required a wool layer (Ice Breaker 200) AND my "bug" cloths (another long sleeve shirt and long pants), wool hat, and then I still had to throw my down jacket over the sleeping bag like an extra blanket to feel comfortable.
At Guitar Lake, my thermometer in my tent said the temperature was in the low 40's, but I could have sworn I saw some frost on the grass as we started up the mountain that morning before sunrise.

[One Half]

I'm currently in MT in the mountains at 7300' and the nights here are already down into the low 40s. It's not the JMT but it IS mountains.

[No Match]

Will I need long johns at night hiking the JMT from 8/23 to 9/15 if I have a 25-degree (actual comfort level) sleeping bag?

This is an absolute. Been there, done that kinda thing. I had frost and ice-skimmed water bottles, frozen log creek crossings befofe ten most mornings on the JMT. I was ever so glad I packed a Capilene Mid-weight set, gloves, & tobaggan plus a 4 ounce backup fuel canister. I summited Whitney freezing to death leaving Guitar Lake at 21 degrees two oclock AM with a 20-30 knot wind. I was a dying penguin. I exited out at Whitney Portal sixteen hours later straightway down to Lone Pine to the Whitney Hostel checking in at 107 degrees. The Sierras are not for the faint of heart, nor gram weenies. To many people from the East(like me) show up ill prepared worring over weight. You can't use what you don't have, but do have the trip of a lifetime.