I've noticed on days that are very gloomy/overcast, i feel sleepy and as if i have less energy than usual. is there a scientific explanation for that?

POSSIBLY: As humans we are diurnal (day) creatures so our system is set up to respond to light. Now if this happen in the fall and winter and lasts then you may have a Seasonal Mood Disorder and require treatment with light therapy.

Scientific data is: variable but some evidence indicate sunlight makes more people happier, possibly by increasing serotonin, which can lift mood and alertness. Psychological factors such as preference affect the effect of weather and research shows some people prefer cloudy or rainy to sunny. Each individual must be self-aware and make adjustments in light and activity based on individual response. Enjoy every day!

Yes: Sunlight has an important effect on mood. This is the cause of SAD, seasonal affective disorder. This is caused by fewer daylight hours and angle of the sun in Winter. Phillips makes a theraputic light "GoLITE BLU", $99 at Walmart.