order by date desc but NULLs last

I have a simple query that sorts by descending date but the NULL dates
show up first. Is there a way I can sort so they come last without
sorting ascending?

SELECT submittime,score,gradedtime FROM student_gradedmaterial WHERE
gradedmaterialid=3 and studentid=102 order by gradedtime desc;

submittime | score | gradedtime
---------------------+-------+----------------------------
2003-01-30 22:56:38 | |
2003-01-31 03:42:29 | 99 | 2003-02-06 14:21:43.043587

but what I want is all the graded items first in gradedtime desc and
NULL afterwards. I do need to keep the NULL score rows. (So I get the
latest submitted grade for the assignment but also any ungraded
submission information.)

Thanks for any advice.
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AgentM
agentm(at)cmu(dot)edu