I have no idea if they will put in more flights, all you can do is keep checking, if nothing comes up you have to bite the bullet and travel over night or have a rather long trip.

Till the 30th of April there is the data and time I need, from the 1st of May it changes. I think it is strange, because July is a very touristic month and having only one flight por "night"...
I think some airlines have a schedule change during spring time, so it's possible that there will be other flights later on, so you have to keep checking.
If nothing comes up I would take the 8+ hour TAM flight, I think that was a day time flight.

Thank you very very very much for your help!!! What I'll do is to wait till April, to see if there are different departure times for May. I'm sure that there will be. In the high season less flights than during the other months???
Very kind of you!

Sao Luis to Manaus is a lightly traveled air route. There will be few flight options at any time of the year. And, in fact, Brazilian airlines are cutting back number of domestic flights by 5%-7% in 2016 due to the economic recession here in Brazil.
For example, TAM just cancelled its nonstop flights between Manaus and Fortaleza and even between Manaus and Rio...
I would doubt very much that you will find airlines adding new flights for those routes in 2016.
using Brazilian travel websites, the best flight (in terms of total flight time) is indeed Gol (Sao Luis-Belem- Manaus) taking about 5:20 hours... Azul would take 9 hours... and TAM much longer...
[pay attention that Manaus is in a different time zone from Sao Luis... it is one hour behind]

You are right, rokatoo, that's happening everywhere in the world, I think.
But why, if you search now in Skyscanner, Kayak, everywhere else, for flights for the 4th of August it gives you more than one option? And then, after filling every single box you are told that the tickets are sold or any other strange thing?
The trip you recommend me is the one with departure at night?

i am not an expert on skyscanner and kayak, but i think i read somewhere that they do not necessarily use real-time data from all airlines all the time(especially kayak, i think). that may explain why your search returns some flights (or fares, that has happened to me a number of times) and then you cannot really book it.
but i just tried skyscanner and kayak searches for Aug 4 and I got exactly the same flights as I told you before... shortest travel time is Gol flight at night with a connection in Belém (about 5 hours total travel time)....
air travel system in Brazil is based on hubs (like in the USA) with a good percentage of flights being channeled thru these hubs.... and no airline has really important operations in that area you are travelling thru... the closest important hub would be Brasilia (all other hubs are farther South, for example GRU, VCP, CNF airports)... so there are not really that many flight options in the region.
and it is not uncommon to have flights very late at night in certain locations - particularly those that have very little business travel (and Sao Luis to Manaus route certainly fits that description).... also, some airports in big cities ban flights after a certain time (like CGH in Sao Paulo and SDU in Rio), so airlines may want to use planes in other parts of the country at those hours.
you still have enough time, but the main advice is that cheapest prices usually require a 3- to 4-week advance purchase in most cases.
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