Sunlight intensity during a day
Well, there are three parameters of interest to be evaluated to get intensity of sunlight.


Sinusoidal variation of sun vector on earth's surface througout the year
The latitude variation. Which is also sinusoidal
The time variation. Which is also sinusoidal


$$ Intensity = A_0 \sin(\theta - \theta_0) \sin(\pi/2(1 - |t-12|/6))$$

Where,

$$ \theta_0 = A_1\sin(2\pi|d/365.25|)$$

Here $d$ is the day and starts from vernal equinox. $t$ is the time from 00:00 - 23:59. $A_0$ and $ A_1$ are constants. Also when Intensity is negative, obviously there it means darkness.

This is very crude approximation. It assumes circular orbit.