Those are two odd examples of an otherwise well-known expression:

Tuve que volver andando a casa a pleno sol y en pleno verano. (I had to come back home walking under the sun at its peak in the hottest of the summer.)

In the previous example you can see that pleno is used to emphasise what's next. Another example:

Nos entraron a robar a plena luz del día. (They robbed our house even it was full daylight.)

In this case the emphasis is used to note that burglars don't usually do that at daylight but at night when nobody is watching. The examples you give just emphasise the time at which the actions are performed, but I must say that using pleno (or plenas in your case) sounds a bit strange to me when accompanying a time of the day, but it's perfectly valid. It's just unusual, that's all.

Those are two odd examples of an otherwise well-known expression:

Tuve que volver andando a casa a pleno sol y en pleno verano. (I had to come back home walking under the sun at its peak in the hottest of the summer.)

In the previous example you can see that pleno is used to emphasise what's next. Another example:

Nos entraron a robar a plena luz del día. (They robbed our house even it was full daylight.)

In this case the emphasis is used to note that burglars don't usually do that at daylight but at night when nobody is watching. The examples you give just emphasise the time at which the actions are performed, but I must say that using pleno (or plenas in your case) sounds a bit strange to me when accompanying a time of the day, but it's perfectly valid. It's just unusual, that's all.

This expression is present in the DRAE:

pleno, na

Del lat. plenus.

2. adj. Que ocupa la parte central o más intensa de un tiempo, un lugar, un proceso, etc. La bala le hirió en pleno pecho. Juan está en plenos exámenes. Era pleno verano.