Indirect bright light harmful to teak?  

I'm guessing you're in the Northern Hemisphere?
In which case there should be no problem.

Uh, well...
UV damage is cumulative and irreversible. North light, while diffuse, actually contains a larger proportion of the blue end of the spectrum. Try to think in terms of intensity vs. duration of exposure. It's always advisable to limit the overall light levels first and then try to filter out the UV (and IR) from what's left. Curtains, shades, blinds, something you can close easily when you are away, are all good. UV filtering glazing or films will help, too. Watch leaving items too long in one place on the top to help avoid hard "shadows" from uneven fading. Those are nearly impossible to disguise well.
In the end, though, it's furniture. We want to live with and enjoy it. Wood is organic and will change with age alongside us. It's part of the charm. I've seen some badly faded, really beautiful pieces that I'd be happy to own.

Hmm sounds like I will be investing in some solar shades for that window then. Until I get them ordered and installed, can I put a table runner on top as a short term protection?

Has
the sideboard been kept in a darkened room for the last 50 or so years, is it faded at all?
I think you might be worrying a little too much.

Shrugs
As long as it's not in direct sunlight, I would not worry about it.

Me neither
I might a Rembrandt watercolour sketch, but worrying about a sideboard out of direct sunlight is akin to wrapping your upholstery in clear plastic.... In my opinion 😉

What, robert, me worry?
I've got some nice furniture frying in my living room as we speak!
I don't believe anyone wants to live in a cave and was only trying to provide some basic background.
BTW, Schott glass offers some pretty awesome glazing options for your Rembrandt.

No
that was for Farmery.
Thanks for the tip for the Rembrandt though 😉

Toward the front of the sideboard, the top is much more orange-ish and lighter than the rest of the piece. This part is also what's getting more light at the moment. My main concern is to stop further lightening of the top.

Of course. Apologies offered.
One of my primary gripes concerning this forum has to do with the format and how it functions more like a message board rather than encouraging individual conversations. Unless made clear, it's sometimes impossible to discern who's responding to whom.
A user profile would be nice, too. And a way to PM individuals, if they should choose to make themselves available... and, perhaps, an occasional cash prize...

Although
it isn't my usual habit, many posters, here and elsewhere, address a previous poster by name when replying to a specific message. That's the only practical recourse, as I see it -- and it seems to work. I admit it's somehow "pushy" . . .

You talkin' to me,
SDR?

UV light is...
UV light will destroy almost anything over time, or later it's color.
Indirect light of course has far less UV than direct light and to be clear, window panes do filter out UV to an extent.
If you have one piece in direct light and another not, then yes, over time it will fade compared to the other.
We have some rare pieces that get direct light and others that do not - not much point of having the pieces if you try to hide them. And a house without light is more or less a prison.

I've got this lovely sideboard
which has been placed in direct light at some point, the fading on the front panels just adds to it's beauty. The result reminds me of the vignetting that you see on some photographs. However, I do own some pieces that I wouldn't want damaged by sunlight - they're well away from the windows.