Leaving leather in the sun for an extended period is not recommended. Any natural substance will experience adverse effects from too much exposure to sunlight. Leather is a natural material; it’s made from animal skin, and the effect of leaving it in the sun will be similar to leaving YOU in the sun. Premature aging, discoloration, drying, and cracking are the main tragedies your leather will endure from overexposure to sunlight.
Why? Well, it’s the sun’s ultraviolet rays that do the most damage (to your skin and your leather). UV light is powerful and it causes changes on a molecular level. The visible change you will notice first (as early as just 4-6 months of regular exposure) is discoloration. Discoloration in and of itself isn’t necessarily a problem - some folks even do it on purpose to change the look of the leather. The problem is, the change in color indicates a change is happening on a molecular level as well. And those changes are not good.
Remember, being a natural material, leather is extremely porous. This means it can easily absorb moisture - but it also easily releases moisture. UV rays draw moisture from the leather, leaving it dry and wilted. More than that, UV rays dislocate vital natural oils from the leather that cannot be replenished. You can (and should) moisturize and condition your leather as a regular maintenance practice, but reversing UV damage to your leather is a whole different beast.
Does this mean you shouldn’t wear your leather jacket or carry your leather bag on a sunny day? Of course not. But it does mean you need to pay attention to where you store your leather (should be out of direct sunlight) and how long you leave it sitting in the sun in your car (don’t do it).
So, can leather be left in the sun? Can? Yes. Should? Definitely not.