Simple past tense and past participle of sober.(verb)
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Use "sobered" in a sentence
"Her expression sobered slightly, her gaze becoming more watchful."
"His expression sobered slightly, although he continued to smile with faint bemusement while he smoothed a strand of hair away from her temple with his forefinger."
"Her expression sobered slightly, some of the exhilaration fading from her smile."