Just closing eyes are also effective as recovering from fatigue, and the effect of them is roughly 80% the same as one of sleeping.と訳してみたのですが、もう少しすっきりとした自然な表現はありますでしょうか。
Just by closing your eyes is roughly 80% effective as if you had slept.
Just by closing your eyes is roughly 80% effective as if you had slept and allows you to recover from fatigue.
ご質問ありがとうございます。
「 目を閉じるだけでも睡眠の8割程度の回復効果がある」は英語で「Just by closing your eyes is roughly 80% effective as if you had slept.」と言えます。
また、「疲労の回復」の部分も追加したいなら、最後に「and allows to recover from fatigue」という風に言えます。
そうすると、最終的な英文は下記の通りになります。
「Just by closing your eyes is roughly 80% effective as if you had slept and allows you to recover from fatigue.」
ご参考になれば幸いです。
アメリカ合衆国 
