We're essentially having the same conversation 
in 2 different entries. Let's keep it at 緊張
感. This can be revived if needed.

Jim: Fair enough indeed, I generally don't like thesauruses too much myself but I'm finding it difficult to source this phrase formally and felt that it would be my best option.
Perhaps it would be more evident with a concrete example:
- 仕事をやめて自由な時間が増えた反面、緊張感もなくなってしまった
- Even though quitting my job has left me with a lot of free time, I also feel like I've lost my edge

Here's a hinative post where a native speaker explains this sentence specifically
https://hinative.com/ja/questions/5956424#answer-15740577

GG5: 緊張感がない feel no 「tension [tenseness, nervousness]; have no 「feeling [air, atmosphere] of tension.

That meaning is certainly not backed by GG5. 
I don't find Weblio's thesaurus to be a lot of use as its synonyms are often only loosely related. I'd never take it alone as the basis for a translation.

https://thesaurus.weblio.jp/content/緊張感がない

Worth adding because the naive interpretation of this expression is "not stressed out"