I couldn't concentrate, presumably because I was so tired.

我的精神集中不起来，大概是太累了吧。

《牛津词典》

That night I was glad, because I was very tired.

那天晚上我很高兴，因为我很疲惫。

youdao

I meet my friends for dinner or I just go to bed because it's so late and I'm tired.

我和我的朋友们一起吃晚饭，或者我直接上床睡觉，因为太晚了，而且我很累。

youdao

So you would say in the past tense "I lay on the bed because I was tired."

VOA: special.2010.02.11

I was very tired because I did not sleep well last night.

VOA: special.2009.05.03

Ms. BUTTU: I left for a number of reasons, but largely because I was tired.

NPR: Bush Visits Israel on Anniversary

"I went to K-Mac and told her, 'It's all you, baby, ' because I was tired, " Diggins said.

WSJ: No. 2 Notre Dame beats No. 22 Syracuse 79-68

"I'm tired because I'm still not at 100 percent, but I'm sure I will return to be how I was before the injury -- scoring goals, having good games and helping the team, " Ronaldo, who returned to training on Friday, told Spanish television channel Telemadrid.

CNN: Fit-again Ronaldo ready for 'El Clasico'