Object reference woe?

Hi everyone,

Every time I try to run my game, I get this error: " Object reference not set to an instance of an object". Unity says that the error is being caused by this piece of code (at the bottom), but I can't see what's going wrong. There is only one game object referred to in the code, and I did declare it and set it to something.

Thank in advance for your brilliant suggestions and fixed code that I'm sure you're going to send me! The Code:

 var dayBrightness : float;
 var nightBrightness : float;
 var dayColour : Color;
 var nightColour : Color;
 var sun : GameObject;
 
 private var nightDayRatio : float;
 
 sun = new GameObject ("sun");
 sun.GameObject.AddComponent ("light");
 sun.light.type = LightType.Directional;
 
 function Update () {    
     
 //________________________________________________________
     
     if (Time.minutes >= 0 && Time.minutes < 2) {
         //night to day process
         sun.light.intensity = ((Time.seconds + (Time.minutes * 60)) * ((dayBrightness-nightBrightness)/120) ) + nightBrightness;
         Time.state = "Night to Day";
         nightDayRatio = (Time.seconds+(Time.minutes*60.0))/120.0;
         RenderSettings.fogColor = Color.Lerp(nightColour, dayColour, nightDayRatio);
     }
     
     //___________________________________
         
     if (Time.minutes >= 2 && Time.minutes < 10) {
         //day state
         Time.state = "Day";
         sun.light.intensity = dayBrightness;
         RenderSettings.fogColor = dayColour;
     }
     
     //___________________________________
     
     if (Time.minutes >= 10 && Time.minutes < 12) {
         //day to night process
         sun.light.intensity = ((Time.seconds + (Time.minutes * 60)) * ((nightBrightness-dayBrightness)/120) ) + dayBrightness;
         nightDayRatio = (Time.seconds+(Time.minutes*60.0))/120.0;
         RenderSettings.fogColor = Color.Lerp(dayColour, nightColour, nightDayRatio);
         Time.state = "Day to Night";
 
     }
     
     //___________________________________
     
     if (Time.minutes >= 12 && Time.minutes < 20) {
         //night state
         Time.state = "Night";
         sun.light.intensity = nightBrightness;
         RenderSettings.fogColor = nightColour;
     }
 }