space usage of NULL fields

Hello,
   a quick question (I couldn't contact postgresql.org, so I didn't check
the FAQ, but I doubt it would be there.)
  When I create a table with field "nnn float8" and total row size, say
140 bytes, and then I insert a row with NULL in this field, will my record
take up those 8 bytes in physical page (say 140 bytes) or will it take up
less (like 140-8=132), i.e. will it be shortened by those 8 bytes, since
the value is NULL and no information is stored in that field ? ( I *hate*
run-on sentences :)
  In general, what are the savings/overheads associated in PG with
NULL/not NULL fields ? I.e. if will I save the space if I define the field
as varchar(8) instead of float8  ?
     Thanx,
        Jan
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