Animate light

how do I animate light intenstiy over time, in the Play mode

You can animate almost any property on any Unity component (including components you make yourself) using Unity’s internal animation system.

For example, take a point light and attach an Animation component to it. Then, open the Animation window, create a new animation, and you will then be able to create keyframes for the light’s intensity values. After you’ve created the animation, you can play it programmatically, or have it play automatically when the game starts.

If you want the intensity to be based on some in-game value (like how close you are to it, the player’s health, etc), you’ll need to create a script that that programatically updates the light intensity each frame.