I'm not 100% sure if you can (as from what I understand you need to take a boat to Railay from Ao Nang, which may not be the best at night - but I'm sure it possible)... However, I recommend a night in Krabi town anyway - it's a great, quiet town with a lovely river and if you are there on Fri/Sat or Sun, there's a night market worth a look too.
I've posted about our night there on my blog if you're interested …blogspot.com.au/2015/10/krabi-town-journey-…
I guess it's better to go in the morning :)
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