Callback help

I'd try breaking sendForm() and bh.read() out of the dht.read() callback. Not sure if that will help, but it might relieve some memory shortage. Untested code:

// global vars up near the top
var lastTemp = "";
var lastHumidity = "";
var lastLight = "";

function sendSensorData(){
   sendForm(lastTemp, lastHumidity, lastLight);
}

// ........
// further down, in place of your setInterval() section

setInterval(function() {
  lastLight = Math.round(bh.read()).toString();
  dht.read(function (a) {
    lastTemp = a.temp.toString();
    lastHumidity = a.rh.toString();

    // let's give Espruino an opportunity to do housekeeping
    // before sending our form
    setTimeout(sendSensorData, 5000);
  });
}, 60000); // once a minute