I've been wondering if lowering my handlebar height would make my bike ride more efficient. The flat handlebar on my Coda Sport is slightly higher than the seat level. This results in a slightly upright sitting position where I can see around better and have better control of the handle. I bike daily in every weather/terrain and at times when it is dark. Should I consider lowering the handlebar height? Thanks in advance for any insight.

If you want a more aero posture similar to a road bike, lowering the handlebars can get you there. You do add some pressure to the hands when doing so I'd recommend long bar ends for more hand positions. Lower bar height can be accomplished by removing spacers (if any) below stem. Inverting stem. Even going with a lower stack height headset.

Who set up this bike for you? On a stock Coda, if the bars are higher than the seat I have to believe the bike is too large for you or your seat height is not adjusted properly.

The handle is only about 1/2 an inch higher than the seat.

Assuming you have spacers below the stem, it is easy to lower the bar and see how you like it... set it back if you don't.

If your stem has any rise, you can also flip it to drop the bars... also easy to reverse if you don't like it.

Is there any loss of control riding with a lower handlebar? Does the field of vision get reduced? At times I ride in the dark and in every type of weather/terrain.

Only one way to find out- use one of the methods mentioned earlier, make the change, and go for a test ride.