How to switch light cycle from durin day to night without changin plants sex??

OK so Im running a gro room upstairs and is currently at 18-6, ON from 3 in the morning to 9 in aftanoon..
Heres da problem When the light is on during the day the temperature is 26 Degrees C. And at night when the light is off the temp drops to 8 degrees as i dont have a heater and do not want to use one due to cost..
So i would like to change the light cycle to turn on at 6 in aftanoon and turn off at Mid day 12 oclock...so that the light can provide heat durin the night as it is not so cold during the day..
Is it possible to do this without changin the sex? A friend of mine told me to switch to 24 hour lighting for a couple of days then to switch back to 18-6 when desired, is this a good method?
Any thoughts would be very helpful...seeing as im not very sure what to do.

I love to Bunstinky !!

just leave the light off till your desired start time..too much darkness wont shock them...its when you give them more light is when they get confused.

Ok thats sounds like an easier thing to do...
and one more thing... The humidity is only 45 is this bad for the plants??

thats good...

Right what ive done is changed it from turning on at 3 to turning on at 9 which will give it a 12 hour period.. remember my aim is to make it turn on at 6 in the afternoon so il move my way up to that over a couple of days... by daily changin the settings closer, is that better than suddenly puttin it to a full 21 hours of darkness... it seems quite long for the plants to sleep..
quik answers wud b helpful

I'm not 100% sure but I read somewhere on RUI that it's actually beneficial to give the plants 24 hours of darkness before switching to 12/12 to induce flowering.

I'm currently on 18/6 and the lights go on at 5AM, off at 11PM. When I switch I want the lights to go on 3PM and off at 3AM, so I'll actually give them 15 hours of darkness before 12/12. They're used to 6 hours of darkness, so this should let them know that something's changing, and hopefully they will adapt...

Kool Fanx very much for da info.
I will do as i sed then give them 18 hours of dark b4 i switch...

BUNSTINKY