King Darius is overwhelmed with concern over Daniel's fate, prompting him to act with unusual urgency. Driven by a desperate need to know what happened, he wakes up at the break of dawn, rising exactly as the morning light first appears. The primary approach among commentators is that this early hour marks the very beginning of the morning. [מלבי״ם] specifies that this was the earliest possible moment, when the day had just barely begun to lighten.
This early rising highlights the depth of the king's distress. [רש״י] points out that waking up during the early morning watch is completely uncharacteristic for a monarch. Typically, kings sleep in and rise much later, around the third hour of the day. The fact that Darius abandons his royal routine to wake up so early clearly shows his intense restlessness. Consumed by anxiety, he does not linger but rushes quickly and fearfully to the lions' den to discover what has become of Daniel.