6-8 hours is the standard recommendation. Less and they get leggy - stretched out. But if you can't improve it it is better to have morning sun than afternoon.

Dave

For 8 summers, I grew beefsteak and cherry tomatoes right up against the east side of a brick house in NE Illinois. (On Google Maps, the house does face directly west, so that bed received only morning sun, no sun from the south during the day.) I was brought up to prune tomatoes to one stem, so these were narrow plants, and probably planted about 6-8" from the foundation.

They had sun all morning until the sun was overhead and the house began to shade that bed. So obviously this was only 6-7 hours of sun, max -- and much less at the end of the season.

I don't remember them being leggy; they did grow taller than I am, but I'm only 5'2", so that's not difficult! They didn't bear a whole lot of fruit, and the lower fruit tended to be stolen by thirsty raccoons. But other than that -- and a bit of Septoria leaf spot at least one year -- the plants seemed happy.

Besides direct sun and shade, there's also bright shade. For instance, daylilies aren't supposed to bloom well unless they receive at least 6 hours of direct sunlight. Mine used to be shaded by the house in the morning and by maples in the afternoon. About noon they got an hour or so of direct sun. They bloomed quite happily. I used to say, "But it's bright shade" -- and later read that there is such a thing.

So I would say it depends how your tomatoes have done in the area that receives 5 hours of direct sunlight.

Perhaps you could experiment: grow some in the current area, put some others in the sunnier area, then compare them -- and of course report back to us!

Moving them also depends on how established the plants
are. If they've been in the ground for more than a week
or so, I'd personally be reluctant. Digging them up will
disturb the root system and generally knock back the plant
for a time.

If it was me, I'd go get more plants and put them in the new
area too. That way you'd know for sure which place is best!

I'm including a link that Carolyn provided years ago about
tomato root development. Interesting read if you're so
inclined.

Here is a link that might be useful: Tomato root development

Another thing to consider is that there are some varieties that can handle less sun than others. I have found that my cherry tomatoes, for example, can handle having less sun than my larger tomatoes, so I tend to plant them according the the sun requirements of each plant. Laurel's Heirloom Tomato Plants site lists many varieties and categorizes them by their sun and climate requirements. I don't know what variety you are growing but some of her suggestions might be helpful.
Don

Here is a link that might be useful: Laurel's Tomatoes

My tomatoes are in containers so moving them to the new area is not an issue.

I have some cherry tomatoes, Better Bush tomatoes and Florida 91 tomatoes.

I think I may try them in the new area just to see how they do.

I'm hoping they get to looking better. :)

My tomatoes only had full, direct sun from 11AM to sundown in SW OH. 4 plants. I can't handle how many tomatoes we've had. The supports collapsed from the weight....But this was the 1st year in raised beds, and I purchased loose topsoil to fill them. Prior years were in containers placed where there was more sun -- almost from dawn to dusk.

Next year: Rotating to another raised bed with the same purchased soil. This bed will have 1 hour less of direct, full sun (but it will have partial sun for that hour).

We'll see what happens in 2013.

I think shading in the afternoon helps a lot as long as they get at least 6 to 8 hours.

I have been here since 1979.I have huge, 80'tall or more, oak trees on the west side and they shade mine in the afternoon. They get full morning sun and the plants closest to the west side do better.

Over the years, as the trees have grown, the toms have steadily improved. I have very little sun scalding any more. Back in the 80's I had a lot of sun scald.

Another thing that I changed a few years ago, I started planting the rows east to west instead of north to south so that they would shade each other and saw an instant improvement on everything in my garden. It is very hot and humid here, especially in the afternoon.

My tomatoes get from sunrise til 4 pm, when the trees on the next lot shades them.
I know a man in his 60's that plant his tomatoes near a oak tree, so he could work in the evening shade, his tomatoes did fine on 5 hours of sun.