Heatmap is a visual representation of the entire market. This will allow you to view and have a clear picture of what sector are hot and where the positive and negative sentiments are. We look for the bigger the box the better, as this will be the stocks that have Major News or Strong Sentiments within those sectors.
FINVIZ.com Financial Visualizations is a good source for a HeatMap. There HeatMap consist of the S&P 500. You should look at this map at 9:00 AM EST and 2:30 PM EST. This is a just a general rule of thumb, most of the time, not much has changed. But you maybe able to get a second (trade) attempt to the sector sentiment you are currently or and looking to trade in.