Column does not exists?

Ok, I have this table:

CREATE TABLE sessions
(
   "SESSIONID" integer NOT NULL,
   "SESSIONTIMESTAMP" character varying(45) NOT NULL,
   "SESSIONDATA" character varying(200) DEFAULT NULL::character varying,
   CONSTRAINT sessions_pkey PRIMARY KEY ("SESSIONID")
)

Now, when I do:

DELETE From sessions WHERE SESSIONTIMESTAMP < '2010-01-01 10:02:02'

I get:

ERROR:  column "sessiontimestamp" does not exist
LINE 1: DELETE From sessions WHERE SESSIONTIMESTAMP < '2010-01-01 10...
                                    ^
********** Error **********

ERROR: column "sessiontimestamp" does not exist
SQL state: 42703
Character: 28

But if I do:

DELETE From sessions WHERE "SESSIONTIMESTAMP" < '2010-01-01 10:02:02'

It DOES work.

Why the db doesn't recognize the name of the table without quotes?.

Regards,
--
Leonardo M. Ramé
Medical IT - Griensu S.A.
Av. Colón 636 - Piso 8 Of. A
X5000EPT -- Córdoba
Tel.: +54(351)4246924 +54(351)4247788 +54(351)4247979 int. 19
Cel.: +54 9 (011) 40871877