What is your soils temperature? Simply because your air temperatures are dropping and may be averaging in the 40's now does not mean your soils temperature will be in that range, yet. It takes quite some time of low temperatures for soil to cool off.

Soil thermometers are about $9 bucks at most garden centers and are worth their weight in gold. You'll be suprised at how long the soil retains its heat. Even after light frosts. General rule of thumb though is if you're still cutting your grass then it isn't too cold.