If you don't get enough sleep, you are not going to be at your best.

[JaneGothic]

While studying the first conditional in Outcomes pre-intermediate, I found a sentence "If you don't get enough sleep, you are not going to be at your best."

Why do we use be going to for this prediction? As far as I know, we use be going to for predictions based on outside evidence. Meanwhile, it's better to use will for predictions based on what we know, believe, or have calculated.

I think here the prediction is based on speaker's knowledge about lack of sleep, so it's better to use will

 

[probus]

My sense, as a speaker of AmE, is that "will" and "be going to" are equivalent and interchangeable, the only difference being that the latter is slightly more informal.

 

[emsr2d2]

You can use the search function to look for the multiple other threads on the difference between "will" and "be going to". You'll find no universal agreement.

For what it's worth, I find the original quite awkward. I don't know why they used the contraction "don't" in the first part, but didn't contract "you are" in the second.
Grammatically, there's nothing wrong with the original. However, the majority of native BrE speakers would, I believe, use "you won't be at your best". Note the parallel in the contractions "don't" and "won't".

 

[jutfrank]

While studying the first conditional in Outcomes pre-intermediate, I found a sentence "If you don't get enough sleep, you are not going to be at your best."

Before I answer your question, could you first tell me exactly where in Outcomes pre-intermediate you saw this, please?