@jwosnick
Very strange as in my case this library work well with sleep() on battery-powered node.

Look like it's nessesary to add delay(2000); before or after sleep() as sleep mode stops all timers, so
currenttime - _lastreadtime =0
and function returns with no new measurements.

@jwosnick

As your set sensor model

DHT dht(DHT_DATA_PIN, DHT22);

it looks like you use "Adafruit DHT-sensor-library".
It's need dht.begin(); in setup(){ } which is missing in your sketch