I have been on Citalopram 20mg for 2 months and I have just been advised to move up to 40mg

When I have been on SSRI's in the past, regardless of the dose I have always had it in one tablet. I was wondering is it best taking it in 1 hit in the morning or having half dose in the morning and half at night, so I would move to 40mg but have 20mg morning and 20mg evening

is there any real difference or not? its just I take mine in the morning and sometimes feel after work at about 6pm that I could do with something to take the 'edge' off as if my levels were dipping again

What do you guys find best?