import collections


def frequent_sleeper(sleep_tracker):
    """Return ID that sleeps the most * the minute they sleep most often."""
    sleepiest_guard, most_sleeps = None, 0
    for guard, sleeps in sleep_tracker.items():
        num_sleeps = len(sleeps)
        if num_sleeps > most_sleeps:
            sleepiest_guard, most_sleeps = guard, num_sleeps

    sleep_counter = collections.Counter(sleep_tracker[sleepiest_guard])
    sleepiest_time = sleep_counter.most_common(1)[0][0]

    return sleepiest_guard * sleepiest_time