Will taking doxylamine at night last long enough to help with my midday nausea?

Try it and see! One half tablet of doxylamine can be taken along with vitamin b6 (pyridoxine) three times daily for nausea of pregnancy. As a side effect can be sleepiness, an alternative is to take a full tablet at bedtime, and just use the b6 during the day. Try both ways and see what works for you.
Doxylamine. Diclegis, which is a doxylamine/pyridoxine combination, is effective for preventing pregnancy-related nausea & vomiting. Taken at night, it may be effective for 24 hours. But, if needed, another dose my be taken in the morning to prevent midday nausea.