slept soundlyDescribes sleeping deeply and peacefully. The phrase can change form depending on the tense or context, such as ぐっすり眠っている (sleeping soundly) or ぐっすり眠れる (can sleep soundly).
JA: 昨夜はぐっすり眠ったので、今日は元気です。
EN: I slept soundly last night, so I feel great today.