Circuit drawing excessive power in sleep mode (new to advanced circuit design)

Hi, I am currently tangled in a problem where my system is drawing just over 10x the current it aught to when it is in its sleep state. I can confirm that the voltage supplied seems to trigger this excessive power draw as when the battery is at fully charged (4.2V) or at roughly 70% (3.9V) the relationship between voltage and current draw are pretty linear and when it goes below 3.9 it then stabilises at around 60-50 uA (it peaks at around 600uA). I am unsure where the source of the problem stems from as I am quite new to this pcb level troubleshooting. Any suggestions on aspects to investigate are appreciated.

This sounds like there is a device in your system designed for 3.3V power and when you hit 3.9V this device is being pulled beyond its intended operating range. Scour the datasheets and verify operating voltage ranges under all operating modes.

Mr. Wagner makes an excellent suggestion with a high probability of being correct. Along with that the first, and usually the easiest, thing I look for when there are applied voltage curiosities like this in a newly constructed device is component insertions. Check to see that every component is the correct type and value as called out on the schematic. As Mr. Wagner suggests make sure that there aren't any 3.3 V devices powered at 4 V, as well as similar errors. Also check that each component is inserted properly. Do this both by visual inspection of each component and by measuring the applied power to ensure it is the correct polarity. Be on the lookout for a component that has pins reversed, or is installed backwards. Both active and some passive components can breakdown at a few volts of reverse bias and draw large amounts of current, but when the voltage drops below the breakdown threshold the current draw dramatically decreases. Finally, check for floating pins due to bad solder joints or not properly connecting unused pins.

So it can really work