So\Because

[eihab]

because.
right?

[hanky]

because, no doubt.

[mimass]

for sure it's "Because"

[enkhee]

It should be because

[messi19]

It's because, for it gives a reason for going to bed

regards
messi19

[Tedy]

i went to bed because i was tired
i was tired so i went to bed
i was so tired that i went to bed

am i right?

[englishstudentnew]

Obviously it is because.
it express reason simply.

[thuenhanhanh]

i think use because, "because" i never hear use so for this case

[mickel]

i go to bed so im tierd

its wrong

because its right because its given the reason ....

[earm15]

In this case the correct option is "Because".

You could use "so" but you would need to rewrite the sentence and put it like this:

- I was tired, so I went to bed.

Notice that in this case you are using "So" as a linker.

Hope this helps! ..