Light weight headphones

Sennheiser HD600.  if you listen to classic music. Very lite , neutral, and comfortable. Pair with tube amp and it sounds heavenly. 

I have both the Audeze LCD-XC and Stax 009s. Both are heavy, which is the main reason for my short listening sessions. If I could find phones that sounded as good but were lighter, I'd be interested. I have owned lighter headphones but I'm just not comfortable with the tradeoff in sound quality.

Out of the dozen I own the best to have on your head is the AKG 240 .
Light with good sound , not much money, and they still sell well 
after 30 years

On my head right now .

ERA-1s are 2/3 the weight.
Might not be light enough for you.

Meze Empyrean. I even pass out in bed with them on. Very light an comfortable.

I forgot to mention, the Sennheiser HD800 are also super comfortable, and more refined sounding than the 600 series variants.  I haven't heard the HD800s, which are supposed to be tuned a little "darker" than the HD800, but the HD800 are a different sound signature than the Audeze house sound, more neutral and detailed.  Some like it, some find it a little too "analytical".  I think they are great sounding headphones.

The most comfortable headphones I own (I have about 10 or 12) are my Sennheiser / Drop HD6XX - https://drop.com/buy/massdrop-sennheiser-hd6xx.  Changing out the earpads can make them even more comfortable.  Not just comfortable, but lots of bang for the buck.  Maybe not quite on the same level as the LCD-X in terms of detail and bass quality/quantity (I have the LCD-XC, but haven't heard the LCD-X), but very easy to listen to.

If your budget is higher and you like the Audeze sound, check out the Sony MDR-Z1R.  They give my LCD-4 a run for the money in terms of bass and sound quality and are a lot more comfortable.

I’ve got the HIFIMAN HE400S and find them to be very neutral and open sounding.  There’s a review of the LCD-X on headphones.com that compares them to to the HIFIMAN Ananda (a couple models up from mine) and finds them pretty comparable sonically, and at around 400g they’re almost 40% lighter than the Audeze.  As a bonus they’re “only” $700 but can be had open box for $600.  The Arya would be the next step up at about double the price.  Hope this helps.