A Home-assistant automation to dimm the lights gradually with vanilla HA.

home-assistant_autodimm-lights.j2

{% set min_dim_level = 20 %}
{% set max_dim_level = 100 %}

# Calculate sun_dim_level based on sun elevation.
{% set sun_dim_level = 100 - (states.sun.sun.attributes.elevation|abs * 3) %}
{% if sun_dim_level < min_dim_level %}
  {% set sun_dim_level = min_dim_level %}
{% endif %}

# Calculate time_dim_level based on time and reset to max.
{% set current_time = now() %}
{% set reset_time = today_at("06:00") %}
{% set start_time = today_at("19:00") %}
{% set end_time = today_at("22:00") %}

{% if current_time >= reset_time and current_time < start_time %}
  {% set dim_level = 100 %}
{% elif current_time >= start_time %}
  {% set total_duration = (as_timestamp(end_time) - as_timestamp(start_time)) / 60 %}
  {% set elapsed_duration = (as_timestamp(current_time) - as_timestamp(start_time)) / 60 %}
  {% set time_dim_level = max_dim_level - ((max_dim_level - min_dim_level) / total_duration * elapsed_duration) | round %}
  {% if time_dim_level < min_dim_level %}
    {% set time_dim_level = min_dim_level %}
  {% endif %}

  # Use the lower value of the two.
  {% set dim_level = [time_dim_level, sun_dim_level] | min %}
{% endif %}

{{dim_level}}