I think David was supposed to take a later watch, since he has to sleep a lot and then pray in the morning in order to get his spells. So, he would be on last watch I guess?
Cassandra volunteers for first shift & midnight shift if necessary, citing her light-generation cantrips as good reason for her to be stationed at night, and if only one shift is needed, that she should take first shift because {bluff}she's a princess{/bluff} and needs her rest.
She is also willing to take up any camp fire tending duties due to her pyromania.