Superlative form of sleepy: most sleepy.(adjective)
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Use "sleepiest" in a sentence
"Don't be fooled by the options market's best-known sentiment gauge, the Chicago Board Options Exchange Volatility Index, which finished the year close to its sleepiest readings in months."
"Amit Tandon Expenditure cuts are not popular, so the National Rural Employment Guarantee Act and a host of similar schemes won't just stay but will likely be extended to India's sleepiest districts."
"February is perhaps the sleepiest month of the year on the national sports calendar, especially when there's no Olympics."