帮我检查这篇英文论文的语法和拼写错误，并提供修改建议。
原始论文文本(例如，本文所述研究的主题、重点和方法):
"The purpose of this study is to investigate the effects of caffeine on sleep quality. A total of 50 participants were recruited and randomly assigned to either a caffeine or placebo group. Sleep quality was assessed using the Pittsburgh Sleep Quality Index (PSQI) and the results showed that caffeine intake was associated with poorer sleep quality compared to the placebo group. These findings suggest that caffeine should be consumed in moderation to avoid negative effects on sleep quality."
Overall, the text seems to be well-written and free from major grammar and spelling errors. However, I do have a few suggestions for improvement:

1. In the first sentence, consider revising "effects of caffeine on sleep quality" to "effect of caffeine on sleep quality" since you're talking about a single study.

2. Add a comma after "placebo group" in the second sentence for improved clarity.

3. In the third sentence, consider revising "intake was associated with" to "was found to be associated with" to make the language more formal.

4. In the last sentence, consider adding "that" before "caffeine" for improved grammar.

Revised text: "The purpose of this study is to investigate the effect of caffeine on sleep quality. A total of 50 participants were recruited and randomly assigned to either a caffeine or placebo group. Sleep quality was assessed using the Pittsburgh Sleep Quality Index (PSQI), and the results showed that caffeine intake was found to be associated with poorer sleep quality compared to the placebo group. These findings suggest that caffeine should be consumed in moderation to avoid negative effects on sleep quality."