Would there be any reason why a wood clarinet would get stuck together when left out assembled on a stand for the night, as opposed to during the day?
In some situations I have been required to assemble the clarinet in the morning, then play it periodically throughout the day, occasionally needing to leave it out on its stand for the night in my room. I know it isn't really a great idea, but many times I have no choice due to the way my day flows.
On days when I dissasemble it before I go to bed, the joints come apart easily. However, when the clarinet has had to be left out before, come morning I can't get it apart.
Does anybody know why that could be? Is there a way I can prevent it?

Yes, any extreme temperature (which is relative to the instrument). So, depending if you have A/C or whatever in your home, between 2 and 4 am the temperature does a grand temperature change, and sometimes a barometric pressure change. This will cause contriction on yoru instrument.

Temperature and Barometric pressure will cause sensitivities in your instrument.

More basically I expect it is water collecting in the sockets & the wood is swelling. Always take the clarinet apart after playing & swab it out taking care to dry the sockets as well as the bore - also if doing a long session do this when you have a short break). If this becomes a real problem & occurs during normal playing any decent technician will be able to sort it for you.

the only way you will ever prevent it is taking it apart...its a drag sometimes but its best for the instrument

I agree with the other posts. Take it apart when your done playing it. The cork is swelling from the moisture! If this is a wood clarinet, the clarinet could be breathing and changing slightly in diameter. If the clarinet is indeed wood, it is prone to crack the sockets. Dry your clarinet and if it is wood. Oil it with the organic oil to maintain the wood stability.