Try a gradient filter on the image. Not knowing the total composition of the image doesn't help guide a creative concept, but this is a common means of getting a look. Nighttime is usually interpreted as being blue rather than darker. Shadows are more prevalent in night shots as well, creating extreme angles, depending on the creative aspects. Direct light is rarely used for nighttime looks, diffuse and slightly softer light are more common.
Play with grades and gamma to create harder shadows on faces, use a touch of blur or glow on the face or other light areas to define light and blue to define dark areas. But I'd start with a gradient.

Thanks so much for both so interesting replies. I have to shoot a scene on a little boat on the shore at night. I was thinking on shooting during the day to avoid the hassle of bringing lights to the shore. I'm going to do some tests with your suggestions to see what can be achieved. Thanks so much!

Thanks Spot - This is exactly my situation. I've got a clip I really want to use. It is shot at twilight. But I've elected to place amongst other clips that are a few minutes further into dark night. If I wish to go ahead I need to "darken" the sky and keep the subject "lit". Yes I can leave it alone, the viewer probably wouldn't see the join - it's me. However, in striving to get the best from this s/w I've taken onboard your suggestions with Gradient. I'm almost there.

What I want to ask you is the another approach/thought I had. The subject is fairly well "outlined" by this twilight sky. I've considered and attempted to produce a Mask of the actual subject, then attempted to do a Parent/Child, and reducing the twilight sky brightness. So . . .

1 - I haven't got the tracks correct to do this. What should go where? And what Parent/Child relationship do I go for?

2 - What settings should I experiment with to get the Masking to do the job?

Grazie

Solid color/gradient will allow yuou to build a mask with transparency oni a target...Then addtion black and noise should build the stars for you.

Here's something you'll want to watch out for. Similar techniques were used in many of the old spaghetti Westerns (and others). Many, many scenes have the problem that shadows cast by the sun (OK, when the sun is blocked by an object) cannot be dealt with well enough to make it credible. Maybe on a night with a very full moon and no clouds there will be some shadowing, but not like you see with the sun. So watch out for this. If you haven't shot the scene yet, just be very careful about how you set it up.

JCG

I understand the Stars thing . . neat! It's the first part I'm having a problem understanding, "Solid color/gradient will allow yuou to build a mask with transparency oni a target" .

On the Parent/Child thing - is it this:

Track 1 - Mask

Track 2 - Original Video

Grazie

Done it!

G

Spot's suggestion is very much like replicating a graduated neutral density filter on the camera lens where the top half lets in more light than the bottom half.

It's basically the same effect, yes. Just more control given that it's in post. Glass is always better.

Then you should go see this Truffaut movie, Day For Night

The French call this effect, "La Nuit Americaine," the American Night

Or you can have some running text crawl across the bottom of the page that simply says:

"it's night....it's night.....it's night....don't believe that it's day....it's night...."

After seeing that - anyone who is looking at your footage will be tricked into thinking that it is night!!

Subliminal stuff is da bomb!!!!